They're analogous to the far right is the main thing. Anarchism/communism/etc. is the gateway to such views. Most lefists don't go that far (good) but some do. Same thing with the far right, they start off as libertarian, ancap, or run of the mill conservatives etc. and end up going into cuckoo land after they watch too much cable news and facebook conspiracies.
In the USA, we have an environment where it's far easier and more beneficial to those in power to co-opt people into right wing extremism than left wing extremism, hence the outsized representation. You can definitely find countries where the opposite is true, it's a fairly big issue in south american and southeast asian nations. What's interesting to me is that the end goals are nearly the same, which is to implement an authoritarian state where there is a powerful insular ingroup that can exploit the masses to their benefit.