Let me give you a practical example of how easy it is to miss something:
During pCARS1 DLC testing, I noticed something wrong with one of the upcoming cars. It was a graphics glitch, nothing in the slightest bit serious (let alone game breaking) but it would have been picked up within minutes by somebody, given the number of people who would have tried the car out. The reason it was hard to spot was because you had to be in a particular camera view, you had to be cornering at speed and it only occurred with 2 of the many liveries available for the car.
So now imagine how much work it takes to test every car, in every livery, in every camera view, whilst stationary, whilst accelerating, whilst braking, whilst cornering slowly and whilst cornering hard. That is an enormous amount of work, just to look for graphical glitches. Then you need to have more than one person repeat it, because almost certainly the first person will have missed something. And for good measure, you get another person to triple-check, because that's the only way to be sure.
That should give people pause for thought as to why some bugs invariably slip through the net. Oh I know somebody will no doubt argue "well, if the programmer did their job properly....." but that's hugely disrespectful to programmers the world over. None of them set out to get things wrong, they're all human, mistakes happen. Hopefully, those mistakes are picked up (as was the case in this example) but it is all too easy to miss them.
Oh and bugs are different to design choices, which seems to be the main focus of your post.
It is perfectly understandable having plenty graphical glitches, especially considering the huge content of the game. I also still see a lot of them, in various tracks mainly, but yeah, it's impossible to fix them all and as you said they are nowhere near game-breaking, just very minor inconveniences.
But having the game still doing pitstops with the opposite actions than assigned in your pit strategy, getting bus-like steering ratio with forced default setups, assists resetting themselves in the beginning of races, leaving you wondering why the car suddenly handles so strangely, chat messages disappearing, people not being able to "see" certain servers in the online browser, cars stuck for a long time in the pitbox, host migration problems, and so many other similar problems (mainly multiplayer related) are not so understandable IMO.
Most of those are either easily reproducible, or happen way too often to go unnoticed, and I think most of them are close to be considered "game breaking" problems, so I would think they'd take high priority and be fixed eventually. At least the majority of them...
Btw, my first post here, hello