Inter-team collaboration drops in a competitive environment. One of the nice things about hackathons is that you can get feedback and ideas from other teams in those environments. If you're actively competing (and for something as scarce as a YC interview spot), then the incentive to help someone else out decreases.
This is not a iron-clad rule—just look at FIRST Robotics, which has managed to maintain a very strong sense of inter-team collaboration even in a competitive environment. Just because something isn't often done doesn't mean it can't be: the rules just need to be right.
As someone who took part in FRC this year and last year, I'd agree that the inter-team collaboration is definitely very strong. We got a lot of help during competition (and occasionally helped others too).
But part of that might be because during qualifying rounds, you are on teams of 3, which change every game. So by the end of qualifying rounds, you play with (and against) most of the teams there, so it makes sense to help the other teams (since you will probably play a game allied with them sometime).
The other reason is that the culture of FIRST emphasizes those ideals, and so they're maintained that way too. One of the core values is Gracious Professionalism, and it really is emphasized everywhere.
I'm not sure which factor is the most important (I suspect it's the second one), so I'm just thinking out loud, but I thought I'd get some more information out there. But I think the rotating teams gives strategic value to helping other teams, which also increasing collaboration.
