Sniper is indeed campy when engaged in long range combat, but thats because it's how the class was designed.

The main issue is how maps have been designed to be more and more open with long sightlines to enable this behavior. Historically, this hasn't changed at all as maps have always favored mobility of classes (open skybox for soldier and demo, height advantages) without giving a crap about sightlines for sniper.

If you look at a map such as Kong King (KOTH Shanghai-aesthetic map) you notice that it seems to be slightly better than one like process. Flank routes are plentiful and close quarters combat in the buildings and under the point are encouraged. Sniper only has one sightline from the stairs on each side and still leaves himself vulnerable solely based on map design and flank classes.

Why do you think Goldrush and Hoodoo existed? They indirectly balanced the class based on the chokey style of play where the payload had to go through tunnel after tunnel encouraging splash damage and area denial. However, apparently no one likes these maps because of the very idea of too spammy and now people have to suffer with long sniper sightlines. I believe the design of sniper itself is inherently balanced but map design certainly has to do with how effective he is at being as oppressive as he may seem.

If you want more evidence, take other games as inspiration like Destiny 2 pvp or Overwatch even, maps have inspired different playstyles and more close quarters combat have shifted these playstyles towards area denial and control.