I imagine part of the issue is that it's simply been a good few years since people have had the last main Bethesda game to play. Both Assassins Creed Unity and Arkham Knight were released only a year after their previous entries, so the gratification and anticipation of playing them wasn't as high as playing something like the new Fallout - 4 years or whatever since the last one. And, as a result, the disappointment of seeing bugs and glitches in those games was greater than the tempered excitement of playing them. Even relatively critical reviewers such as Bunnyhop have made a similar point: that it's been so long since we've had that 'Bethesda detail' that the excitement of experiencing that again does quell some of the longstanding issue we almost expect from this studios games. I think this ties into a lot of the other points reviewers have made, that this is Bethesda's last chance. Angry Joe made this pretty abundantly clear where he said that he was happy to give Bethesda a bit of a pass this round because it was so great to simply play a new Fallout game again, but that it wouldn't be acceptable next time.
Whether that happens or not will be kinda interesting, because I think I remember lots of reviewers saying the same thing around the release of Fallout 3 but only the odd publication (such as EDGE) came down on New Vegas for being a bit of a technical mess.