That they exist, and that employers use them for "team building".

I agree in most cases. For 1-day hacks most people just tend to do solo projects so there's not really much potential for team building going on.

A short hack time really doesn't lend itself well to collaboration.

But I have had great experiences with the colleagues I've gone on weekend trip hackathons with. In that case it did work.
Went with someone from a different office working for the same company, and after that we kept talking which lead to some more collaborations between the offices.