Games were far simpler back then to code as well.So for me, at the time, it was more because there was no choice, rather than the studios/Devs were more serious/decents. But maybe I'm wrong
Not having to worry about 3D made easier to code AI, even for fake 3D like Doom, monsters travel and use only the 2D space to make movement and planning.
You also didn't have units pulling out gadgets or swapping guns, animations were simpler and there was 2-3 recorded voices for each unit.
Pretty much summarized in otherwise shitty video in first minute of watching (the rest is quite garbage tier material):
When you have to do with constant array of variables influencing units, player positioning, 3D space to move, some kind of inner logic to switch strategies, with animation engine running in the background and trying to make everything smooth, it's no accident that once a while you see one of these getting stuck from unknown variable entering to the picture through error.
I mean computer code in itself is prone to errors in physical properties. Most of course are filtered out by another layer of logic, but eventually something weird is going to happen eventually with the amount of calculation happening in screen.
When computers calculate binaries 0001 + 0001 million zillion times, sometimes you come out with 0011 instead of 0010, because the computing unit are so tiny that electrons start to occasionally glitch into other side of equation (called quantum tunneling if I'm not mistaken). The more calculations you do, more natural errors you're prone to get, especially in highly intensive 8 nm GPU processors.
I'd bet even in NES era if you just run the emulation of lets say Ninja Gaiden for 100 000 hours in some advanced AI that was able to test all the possible inputs and outcomes of every corner, we would find some new ways to win the game in lets say under 2 minutes. If you check how ingenious some of these speed runners are, they're able to find multiple exploits in 2D games where for example in Castlevania Aria of Sorrow (GBA) you could finish the game in 15 minutes which should be impossible even for a God to do it in legit way.
And it's the same with even polished ultra highly tested games like Zelda Breath of the Wild where in manner of few days, people were starting to clear the game within just few hours and it took less than a week to narrow the clear time below 1 hour, and now the record is something below 30 minutes. I'm quite sure Nintendo didn't even expect anyone clear the tutorial area in that time even while speed running.
Considering how large the games have become, it would take the size of 5-10 times larger to release games in absolute polished flawless condition, where everything is extremely well tested.
Unfortunately we happen to live in capitalist system where spending insanely more resources to make experience worth for the 5% who have to suffer through worst of the bugs or are going to be the ones to abuse exploits. And I would rather have competition of ideas rather than every company trying to reach some sort of flawless status.