The US has a strong history of antitrust laws and splitting up companys for free markets sake. The globalization as well the digital markets change this, and its perfectly rational for the US to do nothing. Breaking up big tech would be in the interest of the consumer due to increased competition, but its not in the interest of US citizens. In the digital world we live in, competition could come from within the EU or China and that's worse, also due to geopolitical interest of the US. It is just smarter to live with the few oligopolists, as long as all of them are American.