They own the front, middle, and back of the whole thing, and strangle (even if they have to acquire) any other business that tries to break in and get a better deal for publishers, or advertisers both.
Take a read through this, it's a good read: https://www.justice.gov/opa/press-release/file/1563746/downl...
(It was especially disheartening/confirming to see them outright admit (in quotes the DOJ acquired) that they bought my former employer (AdMeld) explicitly to make it disappear from the "competitive landscape." When I worked at Google, I seem to recall having to take yearly training courses on how I should say stuff like that, but these people... felt free)
(I seem to recall that was about Facebook at the time, not Google, but same same.)