Why is the default virtual terminal number 7?

It's not really anything special, and it's not like anyone person said: "YES! TTY7 will forever be the starting point".

Typically when you boot init (upstart) will reserve the first 6 TTYs for text terminals - which leaves the next unallocated TTY, 7, for whatever comes next. Henceforth TTY7 is used when X actually starts. This can be changed to suite your liking in upstart by limiting or increasing the numbers of text TTYs that upstart reserves.

However, it's typically best to just leave the configuration as is.

The first six consoles are text consoles, and X uses the first unallocated console. The number of text consoles is entirely arbitrary, but I remember it being default in Debian 1.2 (rex), so it is unlikely anyone still remembers the reason.