Why is it called root shell?

Because it is a shell for the root user [1].

Or at least the app’s logo is the root user symbol: a number sign [2]

Normal users typically get a $ prompt, while the superuser (root) gets a # prompt [3]

[1] https://wiki.debian.org/Root

[2] https://en.wikipedia.org/wiki/Number_sign

[3] https://unix.stackexchange.com/a/291733