I don't think all these issues are missed in pre-release testing. I think the ones that are found are put in the "we'll deal with that later" basket. Then they tell everyone the issues were only found after release and that they'll fix them in a patch. No developer is ever going to say publicly and plainly: "We just left a bunch of issues in our game because we didn't feel like addressing them before release". They always say they were discovered after release.
A game this large with this many interlocking systems and variables is extremely difficult to test thoroughly.
As Bethesda's statement said, they had 100 testers. Those 100 people never have access to the final finished game. They are constantly testing a work in progress, and as anyone who has used mods knows, even a tiny change or adjust can introduce bugs. Then MILLIONS of people get the game, and of course they are going to discover way more than was missed. It's inevitable. Add into that all the hundreds of different configurations of hardware and software on PCs, and it can become a nightmare of a bug showing up for a few hundred people that can almost never be replicated by someone else.
Hi, former game dev and modder here. The testing conducted on games is extensive - nothing gets on a console without passing rigorous qualifications from Sony and Microsoft, for example. That rigorous testing isn't going to catch rare or minor bugs, though. If Bethesda had 100 testers working 4 years (at 40 hours for 40 weeks), those testers have played the game for 640,00 hours. Bethesda reports shipping 12,000,000 units to retailers for launch. If even half of those copies were sold to customers, the game has been played ten times more in one hour than it has by all the testers during four years. And, as LateWhiteRabbit noted, the testers did not have access to the final game for the majority of this time. I can't tell when Bethesda did a content lock, but I imagine that they've only had access to the "final," game for under a year. I put final in quotes because it would have been changing daily as people fixed bugs up until maybe a month or two ago, and each of those bug fixes had the chance to create a new round of bugs.
Unfortunately, none of Bethesda's test machines has a standard-resolution monitor.