He loves money.
According to Forbes, Gabe Newell's net worth is about 5.5 billion $. That guy doesn't care about money anymore. He can't spend all of his dough in his lifetime anyway. It makes zero difference if he has 5.5 billion in the bank or 11 billion or 20 billion or whatever. There's nothing Microsoft has to offer to him. Plus he hates Microsoft. He literally quit his job there back in the days to start Valve, because he wasn't happy with the company. There's no way in hell Gabe Newell would ever sell Valve to Microsoft. Absolutely zero chances at all.
Minecraft was different as Notch wasn't a billionaire before. For him it was a very tempting deal and I won't judge him for not turning it down (well, maybe a little =P). Maybe Microsoft can buy Valve once Gabe Newell drops out of the company. But I don't see that happening in the next 20 years. If Valve continues to grow until then, Microsoft couldn't afford Valve by then anyway.
There's of course the slight possibility that Valve goes public which would allow Microsoft a hostile takeover in theory. But I really don't see any reason why Valve would do that. They don't need any more money. Their business model consists of high income and very low expenses. No point in raising cash by going public.
Yeah, so all in all, Valve won't get bought by anyone anytime soon.
Official member of VGC's Nintendo family, approved by the one and only RolStoppable. I feel honored.