Valve is a private company. Microsoft can't just buy Valve, Valve would have to agree to that. Considering Valve has (for a company their size) effectively unlimited resources already, and considering that Valve's founder and leader is a known detractor of Microsoft, this is a nothing story. Microsoft will not buy Valve. This is baseless musing, like how I sometimes daydream with my wife about what we'd do if we won the lottery (which we don't play).

Of course Microsoft would love to buy Valve. Just like they would love to buy Nintendo. I'd like to buy a Lamborghini. All these things are about equivalently likely, zero likely.