Because they can be sued for that. Have been sued for that. And while it is possible to reverse engineer this stuff it is incredibly hard to do. So games with smaller fanbases might lack the manpower to achieve it. Or the game was made in such a way as to make reverse engineering impossible.
So why does this law need to exist if everyone is doing it and has been doing it for decades?
Because they can be sued for that. Have been sued for that. And while it is possible to reverse engineer this stuff it is incredibly hard to do. So games with smaller fanbases might lack the manpower to achieve it. Or the game was made in such a way as to make reverse engineering impossible.
Just because it’s possible with a small sample of games doesn’t mean it’s possible for all or even most of them.
Also, even if a normal desktop can’t run a particular game server, there is almost always a way to get a computer that will.