DX12 gives you the tools to make the game more performant, but the benefit is not automatic - Asobo would have to implement it. The MS description literally says this:
“Direct3D 12 is unique in that it provides a lower level of hardware abstraction than previous versions do, which allows you to significantly improve the multi-core CPU scaling of your title”
So lower level hardware abstraction is cool if you actually spend the time switching your code to it, but that takes a lot of time. Also, this is useless unless Asobo intends to make the sim multi-core. Currently, as we know, it runs on a single core only which is why you get those sad frame rates even on 3090 etc.