Probably Obrador is an actual Leftist.

While he's certainly on the left-wing for many issues, quickly skimming through some of his policies suggets that "social democrat" would be far more specific and accurate a description.

We don't have those in the US.

Eh, the US has plenty of politicians that take left-wing stances on social or economic issues, for several reasons they aren't terribly effective at getting things done though.

It's a term used almost strictly as a pejorative.

This is probably the main reason why it is so strongly tied to the "atomic monolith" view; It's easier to view a diverse group of people as 'the enemy' by bundling them into a singular entity.