I've always wondered why, exactly, a handful of companies like Twitter, Facebook and Google somehow dominate the internet in spite of a nearly nonexistent cost

I've always wondered why, exactly, a handful of companies like Twitter, Facebook and Google somehow dominate the internet in spite of a nearly nonexistent cost of entry into the internet and the fact that they, themselves, don't really do much - definitely not something somebody else couldn't easily do. This plays into my conspiracy theory thinking that they're so dominant because they've been propped up by entrenched government interests as long as they pay back the favors when the favors come due.