Because he's a 22 year old big man who is already capable of putting up 20/10. I think some of you are making the mistake of assuming that NBA GM's are going to value Kanter the same way we value him. Look at the situation with Hayward last year. Here's a guy who put up a 16.2 PER shooting 41% from the field and playing a position with a surplus of talent in the NBA, and he got the max.
When free agency comes, at some point, there will be a team with lots of money left and all the big names off the board who is going to look at Kanter, see the production he has on points/boards and look past the poor defense and passing. The same general situation happens every year and every year fans act shocked at the money being thrown at these guys. No reason to think things will be any different this time around.
Yeah but a big man who can't defend....