Being rewarded for being physically attractive or sexually competent is not an obvious barrier to self-esteem. Prostitution is always a degrading act. There is nothing degrading about earning an honest living the best way you know how. Some would regard janitorial work as degrading, but that's not the case either. They may have arrest records which ensure that no firm will hire them, they may lack marketable skills, they may be single parents unable to find affordable day care during working hours, or they may be homeless.
Prostitution is an easy way to make lots of money.
Prostitution is not always easy, and can be downright dangerous. Prostitutes want off the street. While many prostitutes would probably move off the street if they could find an alternative way of making a living, many would not. Arresting prostitutes will keep them off the street. Giving prostitutes police records reduces their chances of finding a job.