[de90]

Because their business model benefits from knowing everything it can about you.

[grumps]

But you willingly allow them to, and in return they give you value back.

[hackinthebochs]

That's the thing, I highly doubt the majority of users are even aware of how much information Google has on its users. Even I as a knowledgeable programmer would probably be surprised at the depth of profiling they have done. And this isn't even considering any sort of machine learning analysis of the data that to extrapolate information not explicitly given. So no, "willingly allow them" is not an accurate description of the relationship.