This is explained in quite a bit of detail in the judge's ruling:
https://assets.bwbx.io/documents/users/iqjWHBFdfxIU/rZ1UlL.0...
Starting about page 197.
> Google understands there is no genuine competition for the defaults because it knows that its partners cannot afford to go elsewhere. Time and again, Google’s partners have concluded that it is financially infeasible to switch default GSEs or seek greater flexibility in search offerings because it would mean sacrificing the hundreds of millions, if not billions, of dollars that Google pays them as revenue share.
[...]
> That was the key takeaway from the testimony of Neeva’s founder and former Google Senior Vice President of Ads and Commerce, Dr. Ramaswamy. The court found him to be a particularly compelling witness. He put it best. When the court asked why Google pays billions in revenue share when it already has the best search engine, he answered that the payments “provide an incredibly strong incentive for the ecosystem to not do anything”; they “effectively make the ecosystem exceptionally resist[ant] to change”; and their “net effect . . . [is to] basically freeze the ecosystem in place[.]” Tr. at 3796:8–3798:22 (Ramaswamy). No one would ever describe a competitive marketplace in those terms. When the distribution agreements have created an ecosystem that has a “strong incentive” to do “nothing,” is “resist[ant] to change,” and is “basically [frozen] in place,” there is no genuine “competition for the contract” in search. It is illusory.