Why would they want to invest in steem when they can just invest a couple of billion dollars and create their own blockchain which they control?

They could do that too. Steem doesn't need so users many to be valuable and, there are people who are looking wider than google these days

And with the coder talent pool Google etc. have already got working for them, Facechain won't be long in coming.

Imagine what happens with Facebook if it added distribution like Steem does.