If being a Marine is so awesome, why is it that there are so many Marines that join the Army and Air Force?

I can easily answer that. It had to do with the Corps enlisted promotion system. I did 4 yrs in the Corps and got out in 1970 with 10 months time in grade as a Cpl. At the time many of lower enlisted in aviation units would have good pro and con ratings but couldn't get promoted because their unit moved around so much. I was assigned to Cherry Point but when I got there the bulk of the unit was doing bombing practice at Roosevelt Roads for about 6 weeks. The unit returned and about 3 months we went to Yuma for about 6 weeks, returned to Cherry Point. The unit would put personnel in for promotions but it took 3-4 months to make it to HQMC and back again. If the unit was deployed then the promotions would be given to a unit (like the supply unit across the street) that rarely deployed. Supply had guys making CPL within a year but aviation unit had LCPLs with 2-3 years time in grade. I finally got promoted to CPL in Nov of 1969 after returning from Nam in June 1969. I had 31 months time in grade as a LCPL.

In 1976 I joined the ANG as a Spec4 and 10 months later (Mar '77) enlisted in US Army. I had to come into the Army as an E-2 in a combat arms MOS (they didn't want most of the prior service personnel from VN at that time). I made E-3 out of basic/AIT. I pinned on E-6 after 2.5 years in a Army Cav unit and E-8 by 10 years in the Army. 5 years later I was selected (and graduated) the USASMA. I never did get promoted to E-9 because the Clinton Administration drew down the Army below 500,000 and my CMF was cut from 6 E-9 positions to 3. I had to retire 2 years later because of the Army's up-or-out policy of 24 years for E-8s. I did 4 in the Corps, 1 ANG and 19 in the Army. Total 23 years active duty.
Marines that join the Army usually make rank very fast. Every day Marine Corps life was harder than Army Combat Arms basic/AIT !
MY HEART WAS ALWAYS WITH THE CORPS ! SEMPER FI

Well, to be a REAL tough person, you need to go submarines. :)

Though the Marine Corps is the only service I had enlisted in and retired from, I can honestly say that the opportunity for advancement is much better than the Corps as a whole. But I can say that I was very fortunate to be in an MOS that had opportunities for me to grow and advance in. After serving almost 13 years in the Corps, I was selected as a Warrant Officer in the Data/ Communication Maintenance field, (2805) and retired as a CWO3 at the MEB level. There are certain MOS fields that have an end of tour for your first enlistment. I don’t know about today’s services because I have been retired for the last 24 years from the Corps. But as I have said before to each his own. Semper Fidelis to all and all for one.

My NCOIC was prior Marine. Had us running PT in formation in the afternoon. Not that far, but the other guys would already be at the club.... We lived.

I tried every recruiting office besides navy and coast guard. Army NG was the most appealing. After 10 years, I don’t regret it.

I served with a 10 year Marine turned AF. His reason was simple. When he was young, single and crazy the marines were exactly where he needed to be. The core was his family and it was all that mattered to him. As he got older, settled down and became a father though it made sense to dial it back. He still wanted to serve but in a capacity more suitable to raise his family. He also said that even though he knows he could of easily served all 20 as a Marine, he coulden't shake the feeling that he could end up being a liability to those around him since his priorities had shifted. He felt it was in everyones best interest to serve the second half of his career in the AF. I thought that was an great explanation. I tell you what though, he was no doubt the sharpest AF NCO I ever served with.

My guess is the Airman's Club. Always booting those fools out.

Being a Marine was awesome but it was very difficult. Four years was enough, then I became a cop, ten years later I joined the Ohio Army national guard. I signed up for a try one (year). Thought that too many people didn't know what was going on including the staff NCO's. I was looking to reclaim some memories of what it was like in the Marines, but could not find that feeling. At the time the guard unit I was in, had motivational issues, too many (student) wanna be warrior types. They shot poorly on the range, did not understand or care about the finer points of marksmanship or many other aspects of combat or policing, since this was an MP unit. At my low rank I was a leader or at least felt like one, I squared away stuff that these college kids would not or could not do. Marines try other branches of the service to see how the other half lives, maybe it's the grass is always greener on the other side of the fence? Glad I saw for myself, I found out just how awesome the Marine Corps is.
Long Live 3/2

I would dare say the reason is quality of life, many Marines that I've met in my lifetime, experienced being in the Marines was no different than serving in the US Navy. Others have moved because their MOS has been deleted with no opportunities for advancement. More importantly, they want to continue to served their in some capacity.

My ANG unit said Combat Communications on the door. If we could get people trained at the first part, we could send them to training everywhere for the second part. At any given time, up to half the unit would be off somewhere at a school or exercise with pay and per diem building retirement points. Even the unit medic had been a Navy corpsman with the Marines. At one exercise that was supposed to be an evacuation, the squadron began issuing rifles and setting up perimeter defense, gaining top marks from inspectors.