Draymond is a small forward playing the role of a power forward. GSW don't need a 6'7 "big". They'd need him more as a wing defender but he's not a great 3pt shooter (yet takes over 4 a game).
If he was on a mediocre shooting team, he'd not get the notice he gets in GS. Sure he adds to the success, but so do the other players.
Lee's contact goes through next season. Draymond's is up after this season. Do you think the GSW will keep Lee on the bench for 16m of cap space, a guy who was averaging a double double prior to being benched, and pay Green a max salary? Certainly some other team will over Green that much. GS doesn't need to. They could get any 6'7" scrappy small forward and be just as successful. So he doesn't fit.
Yeah size matters, and he's small for his role. If they could keep in for free, sure, but teams don't typically pay max contracts for one way players. Green is a great defender, but his offense is just middle of the road. On that team when Steph is pulling a double team and Klay is the rotation cover, Green should score more. 44% isn't shabby but it's not great. He'd do better to leave the 3pt line alone most of the night.