the notion, "the cloud is expensive", ignores the fact that the cloud is not just rented hardware, but staff, facilities, planning, management, etc..

there are businesses where it makes more sense to own hardware and employ your own staff, but if you just want generic compute and storage, you're unlikely to do it as well for less.

also you cannot easily source arm hardware commercially, there is the honeycomb lx2, and its' lead time is months for a single unit. if you want hundreds of nodes, you're gonna use a cloud provider who manufactures their own silicon.