I guess it's the same with some musicians or actors who are introverted or shy in personal contact but give everything on stage. Just an idea although still no explanation.

Possibly because it is more personal in person. It is easier to hide behind electronic devices it provides a safety barrier. In person you are more vulnerable. Especially if you are the only without any clothes. We can also be our own worst critics. Photos and Photoshop don't always reveal the flaws we are able to see on our bodies everyday. The ones only we notice that others don't. This usually always stems in some form of issues regarding image. It can happen for any number of reasons really.