Why? He very well could miss all of next season, what good is that to stay with Golden St instead of signing a max deal this summer, and he will still get max offers. Also, he might not be the biggest fan of Golden St's medical staff and front office right now if they pushed him to play before he felt ready.
But does opting in with the Warriors mean that he's contractually obligated to rehab with their team doctors? Could he just opt in and then stay away from the team and work out and rehab on his own?
That might give him the most control over his future. Who knows what the league will look like going into summer 2020 free agency? Why sign long term now when you can still get the max later plus have more information about where the rest of the league stands?