college is definitely a top reason people join, but anecdotally I don't think it's number 1 (though any data on the subject would be interesting).
It's also very complicated because there are almost always multiple reasons at play, and usually they change as you go. I joined because I was homeless and floating around from crappy and physically demanding jobs, and I wanted to GTFO of where I grew up. After a couple of years though, college became the most important reason to me.
This is so prevalent that the military actually spends a significant amount of money trying to educate young recruits about predatory car loans. Dealerships know they're easy marks and have been funneling them into the debt pipeline for decades.
It's also very complicated because there are almost always multiple reasons at play, and usually they change as you go. I joined because I was homeless and floating around from crappy and physically demanding jobs, and I wanted to GTFO of where I grew up. After a couple of years though, college became the most important reason to me.