Yeah that's a good question...I never understood why some people stick to prohormones rather than just taking anabolics...Plus, theres a much higher risk for the liver by taking prohormone supplements because they must go through you body first, before being distributed, rather than the anabolics who dont have to go through as many steps to kick in.
But to answer your question, I would guess that part of the reason is that the anabolics are much harder to get than prohormone supplements, and you also have to remember that use anabolics is illegal in the United States unless they are used for medical purposes, unlike prohormones. It's a risk that many people wish not to take, so thats where the prohormones come into the picture.

There is no higher risk with PH's as opposed to real gear in relation to liver. It all comes down to the structure and if its 17aa and the like. Don't spread misinformation.