Because…unbelievable. THis is the way how blizzard is trying to make artificial skill ceiling in this game for hitscans and ease projectiles.
Well, a sniper and a sharpshooter are the same thing; the words are literally synonymous.
To be fair, you could even argue that Mcree is a sniper, but that’s splitting hairs even further.
I think the simpliest thing would be to just say that Ashe Widow and Hanzo are all snipers, but their meant to be played at different ranges. Widow being the farthest, Ashe being mid and Hanzo playing mid to close.
No, quite to the point, they are not. They describe two different, if similar, things.
Like how in math class you learned that a square is a rectangle. But a rectangle is not necessarily a square. One is a special-case of the second.
In this case, I believe “sharpshooter” is a term the military uses for someone who shoots from mid-range emphasizing using slow fire and precision, as opposed to someone who uses close-in weapons or fully automatic ones.
“Sniper” is used, to the best of my understanding, from someone firing from kilometers away.
This is the difference between firing precision rounds at the enemy from the backline of the unit, compared to someone firing precision rounds at the enemy from the encampment 
Ashe’s gameplay, weapon stats, all point to her being a sharpshooter. Widow technically doesn’t fire from “kilometers” away. But as far as game dynamics are concerned, we understand she does best when far back, and comparatively, she’s the furthest away from battle. Ashe fights in mid-range, the team’s backline or mid-line area.