People don't have the same opinions as everyone else, and those games aren't utterly perfect. Quite flawed in fact.
Oblivion has a lack of voice actors, repetitive environments, boring combat, bows being absurdly underpowered, the grinding element, levelling up feeling more like a punishment than something good, how easy it is to make your character broken, et cetera.
Fallout 3 is bland, intentionally or no it's still bland. The FPS part of it is incredibly weak, compared to any other FPS fallout just can't compare. Without the RPG parts it would be an atrocious game.
And of course: Their games are absurdly buggy, to some to the point of being unplayable. For games built around immersion, that's a serious hindrance.
This coming from someone who has easily plugged in at least 100 hours to oblivion, and really enjoyed FO3. But at least in my opinion they were much better than Morrowind. I've tried 5 times to play that, and the game always laughs at my efforts.