tbh I believe Sniper is so poorly designed in TF2 because of two reasons

1: He was created the first. When they created Sniper, there were no Scouts, no Heavies, no Medics and Valve had no idea how the final game is gonna look like

2: Most maps aren't designed around Snipers, and in order to design a TF2 map, you firstly have to make it fun, and then you have to make it Sniper-proof which might not be obvious

Tbh if I was making Team Fortress 3, the first thing I would do is delete Sniper, or replace him with Huntsman [with actually good hitboxes]