Who's to say that it's just about the money?
Shaq's still a viable NBA player. Flawed, but viable. Fringe starter (better have someone else who can play 25MPG), but a good offensive player, a good rebounder, and a good defender in some situations and terrible in others. On a decent team, he's a backup. 15-20MPG. Worth probably $4M - $6M/year.
In Europe, who's to say? It's not just the basketball, though... Shaq's about as oversized a character as it gets (in all senses). He does have an ego, yes; who cares? If you don't like players with outsized ego, stop watching Paul Pierce and KG and Big Game Davis and NateRob and every other member of the Celtics ballclub.
Shaq LIKES being the center of attention, and that's fine as long as he's playing team ball (and please point me to any evidence that he was a problem for Cleveland's chemistry).
He's only going to get so much attention if he's playing 20MPG in Boston or Miami; how much is he going to get if he's playing in Europe? He's a guy who loves to bask in the attention, he may figure he's more likely to get that love and attention if he's playing in Europe versus if he's lacing up for ANOTHER season on the bench in the U.S.