They could easily afford to buy EA and Valve but I don't think Valve would let them and if they bought EA I think games will still be multiplat but launch on Xbox gamepass.
EA have a market cap of 36 billion. If MS wanted to buy them out they'd have to pay over that, possible even double that. There's no incentive to sell unless they're actually offering over the standard share price and something like 72 billion is a LOT of money.
As for Valve, they're a private company. Gabe sets the price and if he says he'd only sell for 1 trillion... well Microsoft can't afford that obviously
Bet Shiken that COD would outsell Battlefield in 2018. http://gamrconnect.vgchartz.com/post.php?id=8749702