That’s not how laws against monopoly work.
If memory serves, there were already a bunch of posts in this very thread that indicated the FTC, etc, doesn't get invovled in private company purchases. Perhaps they were mistaken though.
But even if the FTC gets involved, they are kind of a toothless wondering most of the time. The other problem would be that ms buying steam wouldn't substantially change the market.
Here's why:
Right now, steam contols like 80% of the PC online store game sales.
If MS buys them, it will control about 85% of the PC online store game sales. (by all accounts, the MS store sells very little, last figures I heard were around 80% for steam, 5% MS store, 10% Epic, 5% Other stores.
So one entity will still own the lions share of the market. It would be different if MS owned 40%, and valve owned 40%, turning one company into 80%. (net change matters)
As far as mobile game machines, MS has no market share, so adding the steam deck wouldn't change that part at all.
Similar case with the VR stuff.
And Valve doesn't really have a huge software business, so that doesn't change the market substantially either when added to MS's studios.
Also I'm not sure why people say there is no chance this is real, the documents released last year showed the MS was considering both steam and Nintndo, if possible. So not a crazy notion at all. Its extremely likely they have approached Valve, perhaps multiple times.
But it just comes down to if they want to sell. That I have no real idea, just speculation.