Im going to go out on a limb and assume its because sniping in most other FPS games (Ie cod, Bf, whatever) is generally “less stressful” than it is in overwatch.
In a lot of other games- Sniping verges on the egde of being fairly overpowered as a whole. You sit back and 1-2 shot people at ranges where they really cant interact with you.
On top of that character movement is usually limited
Players arent blinking, lunging, flying, dashing, rocket punching, rocket packing, rocket jumping, sling shotting, teleporting- and all other sorts of abrupt movement.
Player models are also usually, relatively uniform.
Once youve got a “corner” held down- You will know roughly where to aim to line up a head shot, and numerous mechanics can easily be turned into muscle memory because of that physical consistency.
In overwatch youve got heroes who are 2-3x the size of others. Some dont even have head hit boxes at times. Some have them on top. Some have them in the center. Some have them in the back at times.
I say this as someone who has, for about 20+ years of their life, played snipers in most games.
The one thing sniping has going for it in overwatch is that youre typically out of range of MOST heroes.
But its very easy for someone to swap onto a hero who can dive you and pressure you.
Sniping in overwatch is “harder” in a lot of regards
The reason i DONT play snipers in overwatch is, honestly- I find it boring. Ive done sniping in so many other genres, why would i want to play a sniper-hero when theres literal “super”-heroes to pick from instead.