It's not built on Ethereum. They're just using it to associate a Galaxy, Star, and Planet to a unique public key (in this case an Ethereum address). Those aren't just "tiers of cloud hosting," they're uniquely identifiable points that facilitates P2P comms (peer discovery and routing) inside the network.

It's annoying that they've built their PKI on Ethereum. I don't like it as well. If it's possible to move it on Bitcoin I'll be one of the first people to advocate for it.

Urbit is a worthy piece of tech in itself and it's merits shouldn't be understated just because its public key infrastructure is on Ethereum.