I have a big problem with thresh and blitzcranks hit box.

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Wait. Are you sure about this? I've fought Lucians many times as Zyra and that never seems to be the case. If my root hits him while his dash is being activated, then it roots him. But I have never actually hit him after the dash begins if he moves out of the way.

Positive.

Well... you are ignoring game physics. Models and hitboxes are 2 completely different entities that the programmers try to overlap the best that they can. The issue is that even though animations from models have travel times, hitboxes doesn't and is a waste of resources to move the hitbox ever so slightly to match the model at every infinity given point. So what most programmers do is either move the whole hitbox at the begging (or the end) of the animation or in long animations they have intervals where they move the hitbox. That is what causes the difference in the overlap between model and hitbox.

Like i said I don't remember the actual reason for the Thresh hitbox, but if you think about it, Thresh hook is a short animation with a relatively long range, so by making the tip wider you could easily save resources and still make it look the hitbox accurate most of the times. it also could be that Lucian dash is programmed to move the hitbox after the animation is finished and not at the beginning of it so Lucian was still at the initial point prior his dash. It could be a combination of boths.

The end point is that you need to balance Clarity and performance, if adding a slight on clarity means stressing your computing power too much then is simply not worth it to do it.