Classic revisionist history. Everyone has seem to have forgotten how underwhelming Johnson looked his last year here. Do you not remember the countless threads about how he regressed or where was the Dakari Johnson from the Louisville game? How he couldn't jump. But suddenly he was going to make these great strides.
Every player is different. But there aren't a lot of examples from Cal's teams that say staying behind is a good idea. Cal doesn't expect these kids to stay so he's constantly reloading. That means you are under competition if you return. Of course I get that you are under competition at the NBA level but when you're a college kid, you see those dollar signs and your first thought is if I'm going to compete, might as well get paid and not have to go to class instead of compete in college and possibly lose more money.
Look at the Harrison's. Both came back to improve their draft stock but it didn't happen. Poythress was a borderline first rounder after his freshman year. 3 years later, he's possibly un-draftable. That's what kids like Briscoe are looking it.
I think in Jones' case coming back did help him, not in draft stock, but in maturity and learning how to play hard 40 min a game. That was a real problem for him his freshman year. For Dakari, he didn't want to compete against Skal and split minutes, so he took a chance and went pro. I think for him, that was the right decision because his skill set wasn't going to improve from year to year at UK. After 7 years of this style, I expect to lose the starting lineup every year.