A git hash means nothing without the repository it came from, so you'd need to distribute both. A tarball is a self-contained artifact. If I store a tarball in a CD-ROM, and look at it twenty years later, it will still have the same complete code; if I store a git hash in a CD-ROM, without storing a copy of the repository together with it, twenty years later there's a good chance that the repository is no longer available.
We could distribute the git hash together with a shallow copy of the repository (we don't actually need the history as long as the commit with its trees and blobs is there), but that's just reinventing a tarball with more steps.
(Setting aside that currently git hashes use SHA-1, which is not considered strong enough.)
And git even has support for "compressed git repo in a file" or "shallow git repo in a file" or even "diffs from the last release, compressed in a file". They're called "git bundle"'s.
They're literally perfect for software distribution and archiving.
People also like version numbers like 2.5.1 but that's not a hash, and you can only indirectly make it a hash.
