You are right it isn't expensive, but it is much noisier to use something like masscan over 16m internal IP addresses if you are pentesting an organization with a decent blue team.

I don't think this tool was made for the use case of HashKnownHosts not being set.

Using shell history, known hosts, netstat, etc are all great ways to find hosts to pivot to.

Substituting ssh with a malicous version is extremely noisy and risky as well.