Well, maybe that's because bio and beth have made shit games for the last 5 years? Just think of the games, how they are copypasted after each other, and you can easily see why there is actually this kind of consensus. Obviously, if the archetype is crap, all the variations on the theme will mostly be crap.
How are oblivion, mass effect, dragon age and fallout 3/new vegas even similar, let alone copypasted ? Oblivion and fallout use the same engine, same for DA and mass effect but that's about it, completely different settings, completely different gameplay etc..
Bethesda and bioware's games are significantly more popular(or mainstream) than any of the other games you mentioned, hence why they get more hate than the others.
And I'm not saying the codex is full of retards and wannabes, I just think that some people really should start thinking by themselves while others who actually disliked said game or thought it was overrated shouldn't constantly keep hitting on it just because it's popular with other people.
I didn't like mass effect much but it is undeniable that it is a really well done game, with great graphics and setting, and even the shooting mechanics felt solid. It's just that it felt more like an action game than a rpg to me but I can perfectly see why so many people outside the codex would like it. I really enjoyed oblivion though, in spite of the main story being uninteresting and unsatisfiying imo.