Because he's a good defender. He's really quite underrated on that side of the floor. No DPOY candidate, but I'd take him on defense over a guy like Wendell Carter Jr., KAT, Vucevic, Zubac, and Nurkic, all of which are on playoff teams or "contenders".
His positioning is good. He's an excellent defensive rebounder. And he uses his hands well to get a lot of tips. He also averages 2.5 stocks a game. That's pretty good.
People assume his defense is bad because he looks slow and isn't intimidating as a rim protector.