The thing is, I believe Forbes reported him to have earned 10-15 million dollars in endorsements last year. Will he be able to get that in Europe? I'm not so sure.
Thus, he may earn more money in the long run staying here, even at the minimum. Combine that with a shot at a 5th ring, Kobe, and a shot to move himself even higher on some NBA statistical records, and he has some compelling reasons to stay here too.