Millians Of Women

I am just amazed at how many women take their clothes off and pose for the camera. There must be millions. And so many do it professionally to, eg models. Apart from money sometimes, I would think the vast majority are amateurs? Why do they do it? There is a big shame element involved surely after a period of time? And then there are the selfies. As the saying goes the internet is forever.
I'd love to hear your thoughts about this.