Why do many people who can name over ten alt-right groups have trouble naming even one alt-left group?

I'm not sure "alt-left" is a thing. I have only ever heard mainstream conservatives from the baby boomer generation or older use this term. Notably Sean Hannity. I think they think that "alt-right" is a slur invented by Hillary Clinton and her ilk designed to paint all conservatives as racists or something and by using this "alt-left" term they can create a category of their own to counteract it. They don't seem to realize that the "alt-right" is self designated so by its adherents and sympathizers and is fundamentally different from mainstream conservatism in almost every conceivable way. It constitutes a distinct political movement in and of itself quite separate from other ostensibly right wing ideologies. To be fair, virtually every liberal and leftist misunderstands the alt-right as well, and they tend to lump in individuals and political groups into the category who have little to no commonality with them. I doubt many people can name 10 alt-right groups.

I'm pretty sure you have some sort of brain damage.

i can't name any alt-right groups.

Antifa is a violent alt left group