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
Well it's not impossible. MS did try to buy Nintendo and Sega once iirc. But Yea I personally don't want it to happen.