Their contracts...
Even if a player ignored that and risked repercussions, the CHL is part of the international hockey system and there would be consequences for the player (and likely the NCAA team that took the player) if they enrolled a player who was still under contract by their CHL team.
The NHL has a detailed transfer agreement with the CHL. I'm pretty sure the CHL also has transfer agreements with the European leagues they select from in the import draft. None of them, nor the IIHF, would look kindly on the NCAA for poaching players with contracts.
Where did I underestimate the NCAA? You're the one getting into it about the supposed qualities of two programs and (inaccurately) asserting that the CHL is a league of 16 and 17 year olds. Not sure why you're bringing up who would win would anyways. The Usports All Stars not infrequently defeat the Canadian World Junior team and likely would also defeat top NCAA programs occasionally as well if they played on a regular basis. They are different levels of competition, one route is not better than the other for all players in all situations.