Ex: if Microsoft buys out office competitors or webmail competitors to shut them down, because those customers are going to be likely to buy more Microsoft stuff.
Buying games studios could be anticompetitive if Microsoft were going to keep them alive and make their output platform exclusive (but iirc they made a commitment not to do that). It could be anticompetitive if they were going to kill them and funnel consumers into their games with continuing development... It could also be anticompetitive if Microsoft were going to give these studios some uberengine secret access to Xbox/Windows that allowed them to make the best games that outside studios couldn't come close to...
But what's happened is Microsoft is killing the studios; all at once is a surprise, but death within like 3-7 years could be predicted. Most of those people will pop up again at other studios or start new studios. Some of them will realized gamedev sucks and do something else. This is the lifecycle of studios purchased by corporate overlords. When key people make enough from the purchase to allow significant but not complete creative freedom, they often make some really good games at their next venture. Too little freedom -> pump out sequels or gacha games; too much freedom -> interesting games that don't end up being much fun.