They are using their power to force competitors out of the market. That's what antitrust is about anyway -- it's not really about how happy customers are. "We're the best!" is easy to say when you've decimated any potential rivals.
The impact on consumers is often used as a justification and evidence of that, but it's not usually a requirement. It is assumed that the act of being anticompetitive itself is harmful to consumers as it limits their options.
Everyone does this, it is called competition.
That is why every free market monopoly that has ever existed has passed and we are not living under the thumb of a single all encompassing mega-corporation and the only pseudo-monopolies possible are the ones guaranteed by law in regulations.