They can and they do, it’s called selling a license to watch. If the platform loses their license to redistribute you may still hold that license to watch with them but since they now are not allowed to show it to you anymore that license is worth nothing anymore. Now if what you “bought” ever enters their platform again your license should still be valid, however often they weasel around this by licensing a “remaster” or other slightly modified version so they can argue it’s a different work.