Not necessarily advocating for it, but mounting stuff isn't always as simple as mount(8). The gnome-mount manpage (which is actually obsolete and has been replaced with something else, but the use-cases are still relevant) reads:
"Mounting a file system into the root file system involves a certain degree of configuration and as such is subject to whatever preferences an user might have. gnome-mount allows the user to control the mount point location, the mount options and what file system to use for mounting a file system. The settings are read from the gconf database (which is per-user) and can also be overridden on the command line using the appropriate parameters."
If you accept that mounting a volume can (a) have user-specific preferences and (b) have specific permissions, then something has to talk to the user session to make those determinations. And a root process reading user settings usually runs afoul of security policy, hence the user-session mounting utilities.