Microsoft needs help when it comes to exclusive games for the Xbox One, and acquiring developers or publishers with a great game selection is the easiest way to get that help.
But Valve is an unlikely source of relief for Microsoft. Valve doesn’t need the money, and few people involved with the company, studio head Gabe Newell included, have any reason to sell. But it’s still reported that Newell owns the majority of Valve, and it often appears from the outside as if Valve is more or less on cruise control. If Newell wanted to focus on something else in his later years, this would be a good chance to make a clean break while pocketing a fortune.