You're like every complainer. "Well doing x doesn't do $, so Riot won't do it." That's horribly incorrect. You're just salty and can't formulate a proper conclusion to your point.
The issue isn't the skill shot. The "Issue" is the champion's hitbox. Hit box are artificially drawn, linear champion markers. Basically, a box. However, Riot doesn't want a champion's model be a point of balance, so instead of having their hitbox be directly their model, every champion has this sort of square a little larger than them. This is to regulate points of contact on champions so that someone like Cass isn't just better against skillshots by design than Sion. Now, of course some champion hitboxes are smaller, it's not to say everyone has the same hit box. Teemo's is certainly smaller than 6 feast Cho'Gath's, however they're all certain scales with each other which is balanced. Hitting a skill shot on a certain angle or part of a champion will do so on every champion, rather than something breezing by Sion's foot hitting him and not Teemo or Jhin's foot.
The way skill shots work is that they trigger the CC as soon as ANY part of that hitbox touches ANY part of the shot's hitbox. This is coded as contact and CC/animation ensues.
There are also small coding instances that can lead to "Bugs". Trist jumping at JUST the right second so it looks like she has leaped from the spot, getting hooked anyways. Kassadin or Ez E/R'ing perfectly so they get grabbed but "jump" out of it.
Don't forget, some skill shots are fatter than others, particularly champions who require this skill shot to do much of anything. A Blitz hook or Morg snare are much thicker than Thresh's hook due to their kits revolving around this initial CC unlike thresh who has utility far past initiation/picks.
and her buggy ult hitbox...
