I think there can be a lot of reasons and each case is individual. But I think that the main reason why people come to this is our environment, circumstances in life and the desire to make money in the "easy" way, when you don't know how to do it differently.
I agree with all your points, but I think you are missing the most important one, being exploited. A lot of woman are forced into prostitution against their own will. Human trafficking is still a huge problem in todays society. I am not very familiar with the subject but I saw a few documentaries about how many woman are promised a better life and then they just disappear each year and are being travelled all around the world just to make some money for the criminals.