At the very least because, whatever its virtues, part of "working at scale" is being able to thrive in an ecosystem of that scale. If you have a system that is fantastic but can't defend itself or propagate itself vs competing systems, then it's not fantastic. Or rather, it is, literally, fantastic: it is a fantasy that the thing can work in the real world, whatever its theoretical properties.

That's my critique against most speculative philosophies in a nutshell, including anarchism. That's my critique against most things, in fact.