How would you easily figure out which version to install? Having a requirement of rsync already installed on the box sounds reasonable, compared to having x86, x64, arm32, arm64, and the rest binaries in the client download...
And that's not counting different OS's, different dependencies such as libc. In my opinion those things would be far out of scope for this project.
I agree that it is beyond the scope of this project but a statically linked (with musl libc) rsync would be possible.