Our anti-monopoly enforcement is actually much scaled back compared to what it used to be for most of 20th century. More importantly, the focus was changed. The original monopoly busting approach, at its peak under FDR, was all about ensuring that the markets stay competitive - it was hindrance to competition that had the feds cracking down on you, not e.g. jacking up prices on consumers.

That was changed under Reagan, and the new approach was that monopolies were only to be regulated if they start to affect consumers. And that's very much by design - according to the people who devised this regime, monopolies are not in and of themselves bad, because they're more efficient due to economy of scale. So, for example, if a merger happens, the reason why they can block it is if they can demonstrate that it would raise prices or reduce quality of service. If they can't prove that, that's too bad - even if the sheer size of the resulting behemoth is obviously a problem for market competitiveness, it's just out of scope.

I don't think it's a coincidence that monopolies dominate the market under these new rules, in a way that they haven't done before.