FreeBSD for example does not have bash installed by default since bash is not part of FreeBSD operating system and when it is installed (using ports or packages) it's installed into /usr/local where all other 3rd party software is installed.

Some systems use /bin and /sbin as directories containing binaries capable of being used in single-user mode (where only / is mounted), meaning a pretty bare experience. root's shell would probably not be bash in this case and bash would probably be optional/a package and would be installed elsewhere.

FreeBSD users have bash added in and so is located at /usr/local/bin/bash