It isn't a matter of leaving. It's that his opt-out is tantamount to free-agency (actually better - he has a guaranteed contract if he opts-in and free agency if he opts-out). So at that point he is free to negotiate a new contract, if he feels that the current contract doesn't live up to his market value. So rather than "why would he leave" the question would be: why wouldn't he try to negotiate a better contract? He may well end up here, the point is, the contract really isn't the home discount it was advertised to be.
To be fair, if he decides to opt out and use the quasi free agency as leverage to get more money or more favorable terms, the team can always say Bye and let him walk, which makes it risky for a player who actually does want to stay here.