if the president is considered the Commander-in-Chief, why do we have so many candidates who aren't prior military?

He isn't "considered" the CINC. He IS the CINC.

The reason we have candidates who aren't prior military is because there isn't a Constitutional Requirement for them to be. It's that simple. The framers didn't think it was necessary, and when the Constitution was ratified, it wasn't included.

But just because someone has served doesn't mean they will make a good CINC. Just because someone never served doesn't mean they won't.

The President also signs things into Law, or Vetoes them. Does that mean there should be a requirement for them to be a Lawyer? He's the Top Level Executive of a 320M person country. What comparable experience can we use to justify that? So we have to use the requirement as laid out. Must be 35, can't be from the same state as the VP, has to be a Natural Born Citizen, and has to have been a resident for the last 14 years.

I could talk about this for days. A few personal opinions.

Political races are all about money, money and money. Who has it? Who's friends have it? And who will it be used on?
Presidential campaigns, really in the last several decades, haven't been run on the premise of experience or results. They've been run on party charters (who can represent a party), and who looks the best in front of a camera. All of the puppet-masters are behind the scenes, and the president themselves REALLY control very little of anything on their own cognizance. In other words, put the prettiest, most charismatic face up there, pay lots of money to get them elected, then tell them what to say.

I would argue that many military leaders (who would be excellent candidates for an office like this) are no-nonsense and tend to be direct. I've heard that state and federal politics is such a silly game, that people of action really get nothing done there. So the good ones don't stick around long.

Think about it. A military leader is used to creating a plan, and implementing it.

Government leaders "Frankenstein" plans together, consisting of a bunch of items that work good in theory, or sound good on the news (will get them re-elected), and are based on a real problem...and then try to ask 300+ congress-people and 100 senators to approve it...that's IF they don't have to get a vote from the largely politically unaware and politically uneducated population of America. THEN, they try to manage it with people who are hired and retained and rarely, if ever, evaluated or fired for performance issues (VA, case in point).
Who in their right mind would want to be in that situation, after having been at the peak of their career as a military leader?

The Constitution; that pesky old document - it specifies CIVILIAN oversight. Of course, in the Framers' time, most were veterans...