1. Risk of injury. You may want to get your money or shot at money before a career-ending or limiting injury. Watching Hayward, George and all these stars have massive injuries - I'm sure these college kids see that and get scared that it will happen to them in college and end their $ dreams.
2. Possibility of better development. After a few years of college you may feel like you have obtained all you can from your coaches and you could grow more as a player by going pro, even if in Europe. Plus, if you aren't in school you can focus on bball all day long and not worry about class.

It isn't like the argument is whether you would trade a year of college stardom for a year of middling pro league anonymity and chump change. It is whether you would trade a year of college for a better shot at major money, an NBA contract/dream, or just a healthy pro career.