John Carmack, author of the Doom engine, is a long time Linux user and for a while the policy was to open source the idTech engines once they had moved on.
However, Doom was hugely popular on its own before this, and was actually more pivotal for making Windows a gaming platform (over DOS).
The reason it runs everywhere is a combination of it’s huge popularity, it’s (now) open source and it’s generally low system requirements.
They don’t, but they define the socket the processor slots into and probably did this to market the newer chips as more advanced than they are (by bundling a minor chip upgrade with an additional chipset upgrade that may have more uplift).
I see no other reason to kneecap upgrades like this when upgrading entails the consumer buying more of your product.