I think it's been proven with the right shooting coach and right work ethic you can improve drastically as a shooter. The guy the Spurs used (not sure if they still do) worked wonders with Parker and then a miracle with Leonard completely reworking his shot and turning him into an incredibly efficient shooter.
Hard to know what goes on for other poor shooters who never improve but I would assume that there's plenty who just give up, never get the right coaching or mentally can't turn things around. You just have to consider the number of incompetent head coaches that constantly get jobs around the league and how poorly some franchises are run and then you can see how shooting coaches may never be a priority.
It's a difficult transition for any player. Drastically changing a players shot will typically involve a transition where that player shoots at a lower percentage than they normally would. It can take months, if not years, to rebuild a player's jump shot from scratch to the point where it becomes muscle memory. That's a long commitment for any player.