It's a simple format easily suitable for accessing resources by byte ranges, that everyone has tooling for, and which _doesn't_ compress things.

Well, according to first paragraph of the section titled "One tarball, served in place":

The whole site is a single tar file. zeroserve indexes it on load - building a path -> byte-range map - and then serves files by issuing byte-range reads against the tarball itself. Nothing is ever unpacked to disk. The site lives entirely in that one file, so there's no document root for a stray location rule to expose, and a deploy is a single atomic file swap.

OTOH, that could be an LLM justification, since the copy is littered with -isms like "the right shape" or "the surface is broad".