They're not KNOWINGLY selling broken games. Hanlon's razor, bro.
Sorry, but I just don't believe that.
They had to know what the problems were with their games before they released, given the amount of problems that exist.
Most of the problems are:
1. Rather obscure scripting issues that you're not going to turn up during a QA cycle.
2. Issues with whatever middleware Bethesda is using, and as such are out of their control.
3. Issues related to stuff slowly getting messed up over the course of the game in really minor ways that eventually add up but aren't something you're necessarily going to notice through usual QA cycles.http://www.uesp.net/wiki/Oblivion:Problematic_Glitches
is a good example of what I'm talking about. I think at least two of those involves involve finishing some quest while another quest is active. Assuming that Oblivion has 200 quests total, if you wanted a systematic approach to finding glitches like that you're looking at at least 40000 test cases. It's not doable.
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Also I think Bethesda's biggest problem has always been a complete lack of awareness about what they're doing -- and then they're catering to a fanbase that's equally unaware and thoroughly in the "games aren't supposed to be fun" camp.
Here's an example: One of the things that Daggerfall had that Morrowind did not was your criminal status being on a per-town basis, meaning that if you were a wanted murderer in Sentinel, you could still NOT be a criminal in, say, Wayrest until you committed a crime there as well.
In comparison, in Morrowind, if you commit a crime in Balmora, the guards are still going to go after you if you run to Ald Ruhn.
In and of itself, this is a cool idea, and since Daggerfall has a lot of towns, it means that if you did something to break the law in one town, you could flee and it wasn't necessarily a "Whoops, time to reload" situation.
The problem is that there aren't a lot of reasons to commit crimes in Daggerfall in the first place, because the only real crimes are murder and theft, and there aren't any tangible benefits for doing either, besides getting into the Thieves Guild or Dark Brotherhood. Which, er, don't really offer any unique services that the other factions don't.
Basically the only reason to steal in Daggerfall is for the sake of stealing and that sort of defeats the purpose of having all of the crime/theft base mechanics in the first place, besides from giving the player a fairly meaningless choice for the sake of doing so.
Also I think doing Dark Brotherhood and Thieves Guild missions just... result in you advancing in rank in the Dark Brotherhood and Thieves Guild. You won't even get any bonuses with the nobility factions, which is necessary to advance the plot, and the only rewards you get from the quests are gold, which... you run out of stuff worth buying after awhile.
So yeah. Two entire factions and several game mechanics that you have no reason to even bother with.
Although the BIGGEST problem is that the quests just aren't fun to do anyway.