It’s called “being 18 to 21 years old”. Some people are mature enough at that age to take full advantage of the opportunities one can have for a well-rounded education…as a number of posts here have already noted (and as I’m about to add), a lot of people aren’t.
I note that you were working on your own degree while in the Army; there’s a level of discipline and dedication which military service can instill in people, which undoubtedly helped you.
As far as my story: I’m another “yes and no”.
I went to a Big Ten school, and did reasonably well in my classes (3.5 GPA as an undergrad, 3.4 GPA for my Master’s program). I took some elective credits which I enjoyed, and which broadened my thinking (several survey-level science classes, several education classes, plus ballroom dance and fencing).
I had a healthy social life, and I made friends who, to this day, 25 years later, are still among my best friends.
But…I skipped class a lot (particularly early-morning classes), and did the bare minimum in a lot of my classes. In a lot of classes, I realized that I could get a B with just a modicum of effort, and I decided that that was good enough. I rarely visited my professors (a number of whom were recognized as being leaders in their fields) or TAs for office hours, and did little to learn more from them.
I very likely spent too much time playing D&D, and not enough time studying. I had serious girlfriends for 3+ of my 4 years as an undergrad, and while I loved both of them, and would never ever regret those relationships, they did contribute to a lack of focus (in fact, serious girlfriend #2 broke things off with me, after we had been together for a year and a half, because she realized that, as a pre-med student, she needed to focus more). I had a work-study job, which made college much more affordable for my family and me, but also took up about 15-20 hours a week…time which also took away from study.