We never got to Hell in D3. The sections in A3 were the Arreat Crater, and the sections in A4 were the High Heavens - both of which had been corrupted and transformed by Hell, but weren't Hell itself. D1 and D2 are the only times we actually go there (and the D4 campaign, obviously). There's the Hell Rifts in A4 of D3 but it was never quite clear whether that's actually Hell or something else.
I mean yeah, but that's the standard PC power fantasy. Did it make sense in D2 that some random adventurer just goes and chases a Prime Evil to their personal lair in Hell and then just bumps them off? At least D3 gave us a kind of explanation by making us this super omega uber being (which was contrived and problematic in its own way), but D4 isn't exactly out of the ordinary here. If anything, it's MORE ordinary, because we have no back story or mission besides just having been in a storm at the right (or wrong) time, and then kinda just rolling with it.
There really isn't a good reason we should be on par with Prime Evils or Archangels or whatever, but it's a video game. There isn't a good reason for a lot of things.