Humans aren't very good at managing things. Just take a look around your co-workers some time and you'll see mismanagement. We're really bad at managing! You could say, scarcity is a management issue. If systems are managed by inefficient beings, well then the system doesn't really matter be it socialism or capitalism. I'll take my chances with capitalism and hope the best managers naturally rise to the top.

Maybe AI can remove the mismanagers from the problem one day. Or maybe, even with our woeful mismanagement, if two hundred governments weren't pitted against one another and we had a single world government, there would be plenty to go around even with humans mismanaging everything.