Even if they aren't all good looking, they are all pretty normal. You rarely see someone that is handicapped or disabled in some way. Extras tend to be young too. So even if they aren't any more beautiful than you would find in your daily life, they create a more unrealistic view of the world. One that doesn't have disabilities.

Totally!
For example, in the TV show How I met your mother, Mc Laren's pub (the place where the characters hang out all the time) is ALWAYS filled with a large number of young and attractive women. This is really unrealistic as well!

Most extras are aspiring actors. Less attractive folks rarely aspire to be actors.

John, this is a great explanation!