The higher a player is rated coming out of HS, the more likely he is to leave early. REGARDLESS OF PERFORMANCE. People need to get that through their heads.
It's probably true that UK is more likely to get a high percentage of players who will basically never stay 4 years, but anyone recruiting elite players is going to go through this. And UK gets more elite players than anyone else. Yeah, you have kids who like college well enough, and see a degree as important (and UK's had those too in Poythress and Lee), but the fact is that a high percentage don't view it that way.
Go back and look at recruiting lists since Cal's arrived. You see a ton of highly-rated guys who left early with either dicey or non-existent draft prospects. Lance Stephenson, Tiny Gallon, Josh Selby, Perry Jones, Will Barton, Quincy Miller, Grant Jerrett, Chris Walker, Cliff Alexander, Cheick Diallo, and on, and on.
Briscoe was 11 in the 2015 RSCI. Gabriel was 13th last year. It's no surprise that they're in a rush to get to the NBA, or at least be a pro.