I'm surprised his agent caved on a non-max deal.
Performance/injury-risk plus risk that a CBA renegotiation alters the max salary downwards, would be my guess. He got about 95% of the max. Also it wouldn't surprise me if he got a large signing bonus. It could be up to $15 million, which would be nearly double Giannis' career earnings to date. It's not insane to not hold out for every last penny.
He doesn't need a signing bonus the contracts are guaranteed and bonuses are taxed much higher so I doubt he would want that. If he needs money that bad he can get a loan against his contract, institutions would line up for that.
1) If you get the money now, it means you can invest it now, which means it's worth more than waiting.
2) Bomuses are very often constructed to avoid state income taxes, and thus are taxed at a lower rate than salary, not higher rate.
3) Getting a loan against a contract would be very, very dumb. Incredibly dumb.