Well they were given theses right by the users who spend lot of money on having these system resources and are asking games to be as beautiful and complex we can (not all the users, i'm not the last to spend time in oldschool games, but a significant and heavy spending portion of them).

Business is exactly why most games dont spend an enormous budget on optimization today. It's not a requirement by the great majority of customers, it's quickly time and cost heavy, so the return on investment is pretty low.

Yes, i think even with infinite optimization budget a today triple A realistic rendering could simply not be possible on a too old computer in realtime.

I also think while it would really add value if background application like teams/slack/discord would be less resource heavy because they are open but not the main focus, when you play a high end video game it make sense to consider it's your main reason to use your computer at that time :)

If simulating and rendering a complete complex intractable realistic but imaginary world with today achievable level of detail seems mundane to you, it's far to seems like to me :)

No opinion about browsers and OS, today games are doing lot more of stuff valuable to most users than those of yesterday. I don't know enought about modern value of os and browser, exept empirically they do seems to crash a lot lot less than 20 years ago, but also syp a lot more on me :)