Mostly because Google is a lot better at it and had far more sources of information about each individual to draw on, for example they have your search history, the history of all your browsing (via analytics), location history for users of maps on mobile, calendar, and email, and docs.

It doesn't take much to imagine what kind of dossier could be constructed on an individual from that data. Quite likely they do know more about many people than those people know about themselves. Of course this can be used beneficially, but it's also the exact information needed to manipulate people.

That is far more than any other service is able to aggregate, and Google's business is fundamentally about mining that data in a way that is not for other corporations.

But yes, the others are a problem too - just on an orders of magnitude lower scale than Google.