as Americans, how can we support struggles that are so far away?

Outside of laundering money to a leftist organization somewhere, I'm not really sure, TBH. Organizations like the International Socialist Alternative (ISA) exist as a kind of way to link up the global struggle across national boundaries. But I have no experience with them and don't know if that's a good or bad route.

And how come we are powerless to change things home?

This is all my opinion. I'm sure there's a way but I think it's pretty well-hidden. Most of human history is not revolutionary but rather the continuation of a bad system. So it's less likely you'd be living in a time of revolution than in a time under some class system that seems to be maintaining itself pretty well.

Having said that, we don't know when the opportunity will present itself either. So in my view, you may as well operate as if a great revolution is just around the corner. Educate workers, organize them, mobilize them, rail against the existing order, etc. The old "education, agitated and organize" addage.

So saying "we're powerless" isn't the point, I guess. Because we won't know whether we are or aren't until we try and either fail or succeed. Rather, I would say a better viewpoint is simply to acknowledge the fact that we aren't currently in a moment of historical change. But despite this, our work is pretty much the same either way. And it's a lot better to understand how political economy works anyway than to remain ignorant and be predisposed to the rat race of electoral politics and the whole spectacle of society.

Like, I can spot the neoliberal propaganda in Marvel movies quite easily. But not too long ago I didn't see it at all. I think there's value in that. I'm growing and learning. And I'm able to understand events in terms of class or class struggle.

Like the issue in Niger right now. As a liberal, I would be wanting to pick sides and know if they're good or bad and probably would have thought the new government was bad and we need to send in the troops. But as a Marxist, I'm like, "Oh, this is like international bourgeoisie against local bourgeoisie and local bourgeoisie has a current interest in decoupling them from neo-colonialism, which is good, but will probably betray that on down the line." Just being able to frame things by class and class interest is useful.

And, of course, in the context of the USA, your greatest impact is at the local level (county). Your vote actually does count there and it's at least possible to form a leftist org that forces policy changes on the local level to materially improve lives. Something along the lines of forming a mass line would be the approach here. But on the state and then national level, your vote is effectively useless. Because those processes are anti-democratic and they're mostly theater to give people the illusion of democracy and make us feel involved when really we're not actually doing anything that nudges power in the right direction ever.