The difference is Valve is completely privately owned, Microsoft cannot force a sale.

With a publicly traded business, the business must be run in the interests of the shareholders, ie it must pursue profits above all else. Thus a buyer can effectively present "an offer you can't refuse", at least the business can't refuse on behalf of shareholders (maybe the shareholders could vote and refuse). With a private business the owner generally has free reign to run the business as they see fit, they could run it into the ground if they so desired.

So it doesn't matter how much cash Microsoft or whoever have, so long as Gabe doesn't want to sell.