Shouldn't be surprising. Porn requires real live breathing chicks. They don't come from some secret abyss. They are AMONGST us! Lots of women do porn and you'd never know it. Or they do it for a while, then keep it a secret.
Right now as we speak -- there are THOUSANDS of men dating women, having no idea about their past. Completely in the dark.
Reasons? Blow. Money. Fame. Self-Esteem. Hot lesbian sex. The reasons vary.