What exactly did they built? AFAIK, even search engines already existed when they entered the game(they just did it better). The fundamental protocols that makes the web work weren't invented by them. What made you think "they built the web"?
I mean thats not entirely true. They build and maintain HTTP3 and Chrome (oversimplification, I know)
They do somewhat control the fundamental elements that make the web work. Blink/V8 is obviously more powerful than Webkit, Gecko, as far as the market is concerned.