The few I went to 10ish years ago had something of a party atmosphere. Free beer and pizza, and the winner was the one with funniest presentation.
Imagine my surprise when they started showing up on new graduates' resumes, eventually I learned that they were more like all night working sessions now.
This is why I participated in only two hackathons, but several dozens of game jams in my life - the "hacking party atmosphere" survived much longer and can (well, could before the pandemic) still be found there. All you had to do is to avoid those jams with official jury panels and non-symbolic prizes, although at times even the presence of jury and prizes wasn't enough to ruin it.
Imagine my surprise when they started showing up on new graduates' resumes, eventually I learned that they were more like all night working sessions now.