How is not rolling your own database a good thing? Mainly because the business of tarsnap is 1) encrypted 2) backups, not building a database storage engine.

Implementing client-side encrypted, deduplicated, snapshot-enabled backups with server-mediated access control inherently requires building a minimal storage engine to represent your opaque log-structured data.

Embedding the log-structured representation of user data in Postgres would increase complexity and overhead without offering significant resiliency or recoverability advantages — in fact, quite the opposite.