I don't think any soap was ever made from olive oil alone. To make it into a soap you need numerous other ingredients in addition to olive oil. Ash is one that I can think of -though I haven't checked this piece of data, as I am writing in somewhat of a rush. The simple reality, however, is that pure olive oil will certainly not lift oily stains off your skin or any other surface for that matter, oil lifting oil just won't work... I have tried putting a little olive oil on my dirty hand and rubbing off the dirt, and as you can imagine I had no success at all. However, maybe I felt dirty as I did not like the feeling of an oily hand, which is because I am not used to it... I don't know; a larger trial is needed.
I am really surprised more researchers of ancient history have not tried to bathe with olive oil. I am sure some historians must have done it, no? After all, historians have tried to reenact almost every imaginable scene from the past, including building mini-pyramids in their back yards, to running with heavy armor while trying to hurl a javelin. I couldn't find the right reference yet, but I am sure someone must have tried.
The dirt that must have built up in the water is certainly a good point. Could olive oil have been preferred -among other reasons- because it floats to the top and could have been skimmed of easily?