Maybe Bethesda is to blame?
Considering that two of the games you've mentioned are from Bethesda Studio, maybe they just need better programmers or whoever fixes the bugs or maybe they should be more attentive when fixing the bugs. But having said that, Elder Scrolls Oblivion and Fallout 3 are both very massive RPGs and I guess it could be a very tedious job for Bethesda to actually eliminate all the bugs and possible to miss some bugs. I have heard people complain about the Elder Scrolls games like Morrowind and Oblivion being buggy in various areas but they're not so major to completely ruin the fun of the game.
I can't comment much on Rockstar because I have yet to experience a major bug in Red Dead Redemption and have never played GTA IV.
