I know tty can be reduced but I don't know why Most linux distro have 7 tty(s) as a default setting.
Why choose 7 instead of 6 or 3? I had never use all 7 tty at the same time.
And, The reason for move X window to tty 7, Two guesses:
For convenient purpose,
To use all tty? Users prefer to start a small order of tty first?