How the hell are Walmart and Target, stark competitors in the same space, both monopolies?
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.
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.
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.
Imagine how awful if we had 5 or 10 monopolies in the same market!