The problem (IMO) is that there is no political pressure for the Justice Department to seek corrective action against these near-monopolies.
The lack of action (IMO) comes from two motivations...
1. The desire for US politicians to see US companies attain global near-monopoly status as a mechanism to help propagate US policy & surveillance.
2. The campaign finance influence these corporations buy through lobbying in Congress and their local/state officials.
1 & 2 aren't even all that independent. Amazon didn't setup HQ2 in DC for nothing. Bezos even literally bought the largest mansion in DC explicitly to host congressional networking events.
The solution is simple, in principle. Nurture, fund, and support alternatives to these companies. Realistically, globalization means there is little hope for regulatory breakups in America's future.