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.
It's easier to release tools for a map based game with no real story. Devs have tools to create content, of course, but making something (tools, APIs) safe and logical enough for the public to consume is a task that can easily get backburnered on the way to release.