All my characters above the age of "was born two minutes ago" have backstories, I just don't always know them.
And neither do I always have to, or at least, I have no more need to know the backstory of someone my main characters meet for five minutes than I have a need to know the personal history of someone I meet for five minutes.
The exception to the rule is Star Wars, where every single thing from the original trilogy has been given a backstory of several books, comics and occasionally video games. It's slightly scary.