Because I really don't see the point. You see him windup, you try to cc him to mitigate the effect, and his hook goes right through whatever stun/charm/suppression you use. How is that not completely broken? And what is the point of there even being a windup? Might as well just make it instant like blitz.
...To dodge his hook? You see the windup, you know where to dodge it. CCing him does not cancel the windup, it just makes him unable to act on it for a bit.
Because if Thresh's grab went out as fast as Nautilus's or Blitz's, he would be so freakin broken. He has that windup so you know to step to the side and avoid it completely.
To warn you the hook is incoming. To compare to blitz grab it's a much longer animation, but does not indicate exactly where it's going. Good Thresh players can use that to their advantage by sending the hook somewhere the opponents don't expect. Whereas Blitz grab turns to face that exact direction.
Try sending Thresh hook backwards versus Blitz grab and you'll see what I mean.
Keep in mind he also has to stand still during this time, so it's an easy opportunity for skillshots on said Thresh.
