Sony is hardly a dwarf company and hardly blameless in doing some monopolistic moves themselves, but they kinda need their gaming division to survive as a whole. Antitrust laws were invented to prevent the actual big(ger) fishes to dominate or take over entire markets, so allowing MS too much leeway isn't a good idea. No idea why only the Brits have said something against it so far, and all others don't dare to step forward.
But the American authorities seem to be way too reluctant if it concerns American mega mergers. Disney becoming this monster is just insane, or allowing Facebook to get Whatsapp.
Allowing Bethesda was okay, together with all other small buys, it made the playing field quite equal, with some time delay to take any exclusive effect, but allowing an even bigger acquisition would inevitably tip the scale in the long term.