There are just lots of little glitches that they could have fixed over time
Hey, thanks for replying
I'd say there are two things about this. [Wall of text incoming, with tl;dr at the end]
One, Bethesda games are compared to other RPGs, but they're not really RPGs, they're simulations, or at least RPGs embedded in simulations. Take The Witcher 3. A beautifully made game, but the world is just window-dressing. You can't enter most buildings. Even in the buildings you can enter, you can't pick up most objects. Most NPCs stand in one place permanently, no matter the time of day or night. You can't interact with them much. Most RPGs are like this: giant theme parks where the attractions and park staff are always there waiting for you to take a ride over and over.
Bethesda's RPGs aren't like that. Todd Howard gets a lot of mockery, but he was right: “you can go there” is the Bethesda philosophy. You can go in the houses. You can pick up everything. You can throw a sweetroll across the room because a sweetroll is a real object in the world, not just a line item in the inventory spreadsheet, and it obeys the game physics like anything else. You can talk to every NPC in the towns. Those NPCs have their own lives and routines. They go to work, they eat meals, they go to sleep—they can be killed. Players sometimes grumble about the “essential” characters in the newer games, but that's only a very mild version of what all other RPGs do. In RDR2, I found Dutch and Micah so annoying right from the very first mission that I wanted to shoot them both in the head, but I couldn't even fire a shot in their direction. If I were playing a Bethesda RPG I could enter the console command to remove their essential status, shoot them both in the head in the first five minutes, and then persist in the doomed world I had created.
With that dual simulation of both physics and society running constantly in the world around the player, it's impossible for there not to be lots of bugs. In a normal “RPG” everything is so scripted that every eventuality can be tested. When you kill that boss there will be a cutscene for the killing blow and the body will be carefully placed in a pre-ordained position. In The Witcher 3 they infamously submerged innkeepers in the terrain from the waist down because they knew the character would never walk around the room and the player would never get a chance to see him. Bethesda can't do that. Every piece of the simulation can interact with every other piece. It's impossible for a QA team to test every potential interaction because the number of interactions is effectively infinite. They cannot launch with no bugs because they cannot test an infinity of possibilities. It sounds like Microsoft has tried to brute force this for Starfield as much as possible by putting all their QAs from across the company on the job, but that entire effort will still be only a fraction of the playtime the customers will rack up within just a few weeks.
Two, yes, they could have fixed more little glitches over time, but it's a question of cost-benefit. As above, it's literally impossible for them to test every one of the infinite number of possible interactions. So they have to prioritise, and the bugs that don't get fixed mostly aren't really hurting anyone.
Take the notorious frame rate bug (if you increased your frame rate via a mod, you increased the speed at which your character moved through the world, because the world clock was tied to the frame rate to make the simulation flow smoothly). Yes, we know they could have fixed it, because they eventually did fix it when it appeared in Fallout 76 and was a big problem because it meant players could cheat other players. But in the single-player games they didn't need to fix it because only people deliberately breaking the game would encounter it, and because it's a simulation Bethesda give the player a lot of freedom. If someone wants to increase the world speed in their single-player game so they can run around super-fast, why stop them? They're not hurting anyone else.
Most of the remaining bugs are like this, obscure interactions that barely anyone actually encounters even when they become famous, and which don't really hurt the players who do choose to encounter them. Fixing them costs dev effort though. That effort could go into fixing most of the remaining minor bugs that aren't hurting anyone, or it could go into making the next game in the series. I think most fans already think Bethesda take too long between games, so the equilibrium they've struck is probably about right.
Tl;dr: Bethesda's games have lots of bugs because they're simulations, and most of the remaining bugs aren't worth fixing.