Why do so many people in the military have useless degrees?

This is a somewhat complex answer to a simple question.
Sociological factors contribute to it.
In the 1950s studies were done that resulted in the saying that a person who had a college degree would earn one or two million dollars more in their lifetime than a person who didn't have a college degree. Even then, it was the STEM degrees (Science, Technology, Engineering, and Math) that pulled that number up over other degrees (mostly the humanities). Guidance counselors have never stopped mindlessly parroting that line. The reality is, many trades will now out-earn the majority of humanities degrees by far.
Second, up until approximately 1980, pretty much any degree would get you a job immediately upon graduation. That's why you'll hear so many Baby Boomers say, "Any degree is a good degree." That was true for their generation, but has since changed. And even for those who can get a job, approximately half of graduates who are awarded their Bachelor degrees before age 25 will go back and get a second degree.
Third, there's a certain amount of residual societal-level misinterpretation going on here. In the old days before things like OSHA and ergonomics and other programs transformed the American workplace, a blue collar worker could expect to have his body worn out by age 55. They would see guys working in less physically demanding positions in management in office environments, and the only visible difference between the blue collar and white collar workers was that one had a college degree, and the other didn't. So in the blue collar world, the attitude became, "You can either work with your back, or you can get a college degree and work with your brain." The assumption was that a college degree was a ticket to a high-paying job that wouldn't kill you over time. That was pretty correct, again, up until about 1980, but has since changed. What hasn't changed is government, media and academia pushing that outdated message over 40 years later.
Through the 80s and 90s the costs of an education started skyrocketing far above and beyond the inflation in the cost of living, and the social programming that harangued entire generations into going to college didn't change. So the percentage of Americans getting college degrees went up dramatically, watering down the value of a degree. On top of that, the number of programs not actually teaching employable skills has exploded, watering down the value of a degree even further. As the joke goes, "I got a PHD in medieval feminist interpretive dance studies. Why can't I find a job?"

So in the end, it's not that an English degree or a foreign language degree or a history degree is worthless. It's that the number of people getting awarded these degrees far outweighs the number of jobs available for people who hold them (plus the fact that some degrees legitimately don't give you any real job skills).

I don't know if people have useless degrees. Either way having a degree of any sort is a blessing. However, some degrees may not fit the lives of others. For some of us seeing the acomplishment of fellow Airmen, Soldiers, or Sailors get a degree is a proud moment for some and a sad moment for other that don't have one. So overall I believe that people don't have useless degrees just degrees that everyone doesn't agree too

It's to show they caan complete something

They have degrees like that because most get online degrees and the options are limited. Unfortunately that useless degree will probably be better for them in the civilian world because civilians can’t wrap their head around our experience equating to civilian jobs.

You're actually asking two questions here--the issue isn't the major--majors don't really matter. I majored in Nuclear Engineering and spent 30 years in the Medical Service Corps.

The issue is with diploma mills, as you call them. Now, that said, I went to American Military University for a Masters in American History and found it to be academically rigorous (I have a PhD from U of MD, a Bachelors from Michigan, and Masters from Iowa State, South Dakota Mines, and the Army War College). So not all schools out there are diploma mills, either. But you have to shop around.

And I agree with Victor Harris--the more we can get people to write, the better. Not all schools do that (the War College included).

Now, are some majors better than others? Perhaps. But a hundred years ago, when Freud and his cohorts were inventing psychology, people thought that was a made up degree as well. Now we can't hire enough of them (at the Masters level, at least) for our hospitals and clinics. Is Homeland Security and WMD a fad? Perhaps we'll find out in another ten years.

I think this is what happens when an organization, the military or any other, sets a requirement for an educational degree rather than for the actual skills needed to be effective on the job. Individuals who believe, correctly or not, that to advance within the military they need a college degree will pursue a degree and maybe choose to do so in a field where getting that degree is the easiest. Working full time, in the military or elsewhere, and trying to earn a degree is a serious challenge, and I can assure you that seeking a degree in something like any of the engineering disciplines is a lot tougher to accomplish that getting a useless degree in "gender studies" or any of the various ethnic or racial studies programs that are now passing for legitimate education.

The military rewards degrees fairly heavily. Since the degree is what matters rather than the content, going after either content of interest to the subject or the easiest will often be the result. With the exception of medical, engineering, or IT degrees, the degree you get will most likely not have a direct application to your job. Certain gen. ed. degrees, like English for instance, are always applicable as they improve the professionalism and conciseness of a soldiers writing. If anything, I'd say an English degree would be one of the most useful for the average soldier looking to make a career out of the military.

My degree is in Chemical Engineering. I've never used it, so its pretty useless to me. The point is that the breadth of degrees offers a similar breadth of understanding, knowledge, and experience. We want to leverage every SMs unique skills and backgrounds to make our Team stronger

I think you have to be very careful about two things. First, calling educational programs degree mills. Before you do that you should understand that an accredited college or university, whether accredited by a regional accreditor or a recognized specialist accreditor is not a degree mill if it offers courses or degrees online. Online instruction is commonly used these days in educational programs from Associates to Doctoral degrees by a wide variety of schools from Research 1s to community colleges. Good online is every bit as good and may be better than in-person bricks and mortar education. Second, interdisciplinary studies, English, and psychology are all degrees with direct utility in the armed forces, in government, and in the private sector. A well constructed interdisciplinary studies degree gives the student a broader understanding of the world around him or her. English - try working on a major command staff if you can't write clearly and concisely to convey the message your four star needs to hear - someone will have to rewrite or trash can (more likely) your gibberish. As for psychology, especially abnormal psychology, many times when one of my troops did something that made no sense, I would have been very glad to be able to understand how his or her brain worked. Mass destruction, terrorism - if you want to progress in a fire department, a law enforcement agency, emergency management or homeland security agency, credentials in these fields will get you in the door and get you promoted.

To get promotion points.