I think they have the "one and done" rule now which means a player must do at least 1 year of college. Unlike the rule in rugby league which is "old enough if good enough".
The college system has awesome atmosphere and crowd support. Some high school teams get better support than NRL clubs. Probably have bigger budgets too lol. I've been told this is because of the demographics of the USA. Not every city has a major league team so people support the local college teams. It's pretty much like following a major team as there is heaps of media coverage, big stadiums, and games are often live on TV.
There are a stack of overpaid prima donnas in the NCAA system. It's why so many want out as soon as possible - get that money. It's why they have agents getting busted for doing deals with HS players and college "pay for play" scandals.
And on the question of putting in every game, just type "college basketball point shaving" into
google and you'll see plenty of gambling trouble where players aren't trying their best.