Avoiding cgo would be a selling point for anyone else using go. Presumably other pure go kv stores like bbolt/badger/goleveldb would also solve that problem, but I don't know enough about them to understand the trade-offs.

Yes, but I think CockroadDB is in also in a position most other people are not: they are a database, so data storage is not only their expertise, it's their reason for existing. They have the people, the expertise, and it's core to their business. Most people don't have that, so they can't justify writing their own storage engine.