His terrible supporting cast shoots better than he does. Pretty much all of them. They're obviously benefiting from his presence, but it's still hard to justify him taking way over 1/4th of his teams shots while missing a higher percentage of those than almost any player in the league, and every other important player on his team.
And it would be easier to buy the "he's only doing what he has to do" argument if he hadn't been playing with and taking almost as many shots as Kevin Durant, one of the greatest shooters in league history, just last year. And not hitting that much higher of a percentage than he is now.
It's just hard to completely accept the "he has to be so ridiculously inefficient because everyone else on his team sucks" argument. No question Westbrook is an amazing player, no question he's doing amazing things, but does he really understand that basketball is a team game, about more than an individual accumulating stats? Would he be happy if you told him that his team would win 60 games, if, and only if, he took only 15 shots per game? I think LeBron would accept that, I think Steph would accept it, I think Kevin Durant would accept it, I know Kawhi Leonard would. Even James Harden might. Not so sure about Westbrook.