You've hit the nail on the head regarding the difference between the EU concept of antitrust and the American one.
EU antitrust law is grounded in harm to companies. If there are many companies, but one has sewn up the market so strongly that others can't really have a chance to prosper, the EU will step in.
American antitrust law is grounded in harm to consumers. The US regulators won't go after a successful ad company because other ad companies say it's too hard to compete with them; it'll go after them if consumers say "We don't have any choice but to do business with X, and X's prices are too high / X is too exclusive / X is only doing business with us in market sector A if we agree to use them also for market sector B."
That additional bar to regulation means the EU may very well have cause to act before the US does if Google is big enough to muscle around other ad agencies and set the terms by which they can exist, even if they do exist.
I recommend you look into it a bit more if you haven’t - there are buyside and sell side competitors, but google massively controls both markets, which it uses to grow both businesses.
For example, the justice department found slides from a project that favored its buyside customers when running supposedly impartial auctions on the sell side - in layman’s terms, brazenly fucking over their competitors. Allegedly.
The best part is of course that the google engineers named it “project bernanke” after the Fed chairman, since they thought of it like QE, I.e. “giving out free money” to people who used their buyside products. I’m still not sure if that was a criticism of QE or if they’re honestly that submerged in the Google Kool-Aid…
All of that is small potatoes to the main way in which google is a monopoly though: they run the only ad exchange. Trust me, it helps. Facebook spent many billions trying to stand up a competitor and ultimately gave up, which should tell you how valuable that privilege is
EU antitrust law is grounded in harm to companies. If there are many companies, but one has sewn up the market so strongly that others can't really have a chance to prosper, the EU will step in.
American antitrust law is grounded in harm to consumers. The US regulators won't go after a successful ad company because other ad companies say it's too hard to compete with them; it'll go after them if consumers say "We don't have any choice but to do business with X, and X's prices are too high / X is too exclusive / X is only doing business with us in market sector A if we agree to use them also for market sector B."
That additional bar to regulation means the EU may very well have cause to act before the US does if Google is big enough to muscle around other ad agencies and set the terms by which they can exist, even if they do exist.