Most dive tanks and DPS need to use all of their mobility just to get to her (that's assuming her other five teammates don't heal/peel/intercept for her). And then she will just grapple away.

Plus let's say you did have a match where a widow felt oppressive, why shouldn't they be rewarded for having good aim and positioning and why shouldn't you be punished for having bad positioning and hero picks?

Well why is Widowmaker the only hero pick that gets to be like that? No other hero has a 300DPS unlimited range hitscan weapon, wallhacks, and a handy mobility tool for escape.

Pretty much every other hero pick has an equal-ish counter - for example, against a good McCree, you could either mirror it or it could be substituted for Soldier, Ashe, Tracer maybe, and they'd all have about the same control of the battlefield as him.

Widow, on the other hand, has no such equivalents. Hanzo is a projectile hero with no scope, Ashe can't oneshot without a Mercy pocket. If your tankline already has their hands full holding down the frontline against the enemy tanks, it falls to your DPS, and if they can't play Widow to the same degree there is nothing they can reasonably substitute to counter her.