Why does GDM run on TTY1 and gnome-shell on TTY2? [duplicate]

I found an explanation on Gnome's Bugzilla, see below. Hopefully they will address the increased memory consumption by tearing down the greeter after user login.

GDM's behavior was changed in 3.16 so that instead of running one X server on the login screen which is then transformed into the user's session, we now start a new X session from within the user's session, which means that we need to start the user's session on another TTY/VT.

Note that when using fast user switching, you effectively got this behavior as well, with each user on a different TTY/VT. We just extended it to the first user to log in as well.