1. Z is one of the best centers in the league right now - which isn't saying much, since there aren't very many really good ones - but he's pretty good, and very good when you compare him to anyone else available. He was worth what they're paying him.
2. Webber's contract is similarly bloated for an equally damaged player - which is why the Kings dumped him. The 76ers were desperate for some offense to back AI up - and they aren't as picky about getting a durable, defensive PF that Webber was once hyped to be...
3. But other than Webber, those other players you mentioned are all relatively healthy. None of them have insurance companies refusing to cover them.
Curry is an exceptional case because there hasn't been anyone else in his position.