It's not for no viable reason. Rust is just safer than C. There absolutely are bugs with GNU coreutils, so it's not even a hypothetical like you implied. But beyond safety, some of the Rust equivalents are more performant than their C counterparts.

And uutils is already heavily tested against the GNU coreutils. It's not some fly-by-night rewrite that people aren't serious about. I don't know if it's been formally audited yet, but it absolutely will be when companies like Canonical (and hopefully SUSE and Red Hat, one day) want to start shipping them.