IMHO appropriate laws should be applied to any company which directly or indirectly affects some "high" threshold of people. Or another way to think about it would be if it's practically impossible to enter a market even with a decent start capital because of it being dominated by a small number of mega cooperation it's not (much) different then a monopoly.
What does it matter if there are 1 or 5 companies which effectively undermine competition in a harmful way. It's either way harmful.
Having 5 companies that actually compete for business wouldn’t harm consumers. Having 5 companies that avoid competition would, and that’s called a cartel and it’s already illegal according to the anti-trust laws (the Sherman and Clayton Acts).
A cartel is illegal yes, but proving a cartel is dam hard. And depending on jurisdiction requires proving that the companies actually had some "agreement"/"communication" wrt. their non-competing (as far as I remember).
But if you look at markets all around the world having situations with the same harmful dynamics as cartels but for legalities not counting as cartels is not uncommon.
Monopolies might be harmful in that sense, but the issue is that monopolies tend to happen naturally as a company grows and expands into a national corporation, which is the most textbook definition of capitalism and the american dream. In general antitrust laws are meant to 'allow' competitors to exist should a new competitor attempt to challenge the incumbents, but they're not an attempt at forcing competition by hurting those incumbents.
It happens because (especially but not only the US) economics system is designed to feature monopolies as it's a efficient tool to economically suppress/control other countries.
And as far as I know some of this laws where meant to force competition. It's just lobbyist have effectively defused/removed/reinterpreted them.
Yes, that's why the biggest companies are constantly lobbying to pass antitrust legislation. Because of the extensive history of small upstart competitors being destroyed in antitrust cases.
