They bought Minecraft for about 2 billion dollars and according to some articles Valve is worth 4 billion dollars. Microsoft obviously wanting to control the PC gaming and console gaming market. This seems like a better choice. Don't get me wrong, Minecraft is huge and all, but valve has the power to single handly win you the console race and keep Windows PC gaming relevant for life. Counter Strike, Half Life, Left 4 Dead, Portal, Team Fortress and of course Steam. Seemed like the better choice for what they want. 

Gabe doesn't want to sell...

What if Microsoft throws in a big mac...

$4bn is too much.

No doubt that there's no interest in selling; nor buying for what they want. It's not just so easy as flashing your cash and, let's face it, why would Gabe want to sell when it's making so very much money?

Looking at other comments in this thread, seems everyone feels the same.

It will have  to be a shitload of them...

What if it comes with the secret sauce formula...

I think a pizza burger would be the better choice here!

Because Valve doesn't want to be sold.

I think you need to read up on Gabe's relations with Microsoft... Valve started up when Gabe left MS when he was fed up with their BS and he is not a fan, to say the least.
Valve will never be sold to MS, I don't see any reason why it should be sold to anyone.