bin/sh shell that is the default root shell is not strictly a "bash" shell, but is a POSIX compliant equivalent. According to this site, when you invoke the shell executable of a "POSIX compliant" shell, it runs any of the "profile files" listed here:
$HOME/.bash_profile $HOME/.bash_login $HOME/profile.ksh ($HOME/.profile on UNIX systems) $HOME/.profile
I've tried putting them all in
/var/root and none of them execute when I run
sudo su from my terminal.
sudo bash works, by the way. But why doesn't