Bash doesn't exist in a vacuum. It's designed to make use of and interact heavily with those external commands. An environment missing dd, head, cut, and tr is probably not going to provide a usable /dev/tcp filesystem, either.

Ultimately the point is that a Redis client is possible using just a standard Unixish userland, and sysadmins who interact with Bash all day can easily make use of redis directly and integrate it into existing scripts without installing special client libraries or writing wrappers in Perl or Python.