Going all-in with 39o, getting called by AA and flopping a boat is the perfect way to get off tilt. Even more so when you're not risking any money. Even more so when you know the guy who you just busted was taking the game seriously and you might have just crushed the dreams of an upstart young pro.

In addition: since it's literally not worth it at all to play in a freeroll unless you make the money, and there are usually very few paid places, people just unicorn (a term my friends invented for pushing every hand of a tournament) and bust out early or have a much better chance of getting cash by building a big stack.

In limit tournaments: sometimes people are unaware that they've registered for a limit tournament and want to get out as quickly as possible. Also, for people who've played mostly no limit there's something funny about capping every street with J high and still being alive in the tournament. It's absolutely hilarious when people fold as well.

In omaha: people don't know the rules of omaha/want to go AI for reasons stated above but can't since it's pot limit.

Basically for >80% of the people in a freeroll it's just a way to waste time/fight boredom so just embrace the fact that nobody is taking it as seriously as you are.