Because he wants to make money now, not next year. Agree with it or not, it isn't that hard to understand.
Any 20-something kid can say "I can always start my career next year". Why would they want to start now instead of waiting a year? Because they don't want to wait a year to start making money. If he wants to go pro even though it's not going to be in the NBA, it means he values a professional life more than college life right now.