The reason all prostitution is rape is because any sex that a woman does not truly consent to or can not consent to, is rape.
Many prostitutes are trafficked and forced to have sex with men i.e. are raped.
Many are dependent on alcohol and drugs and so meaningful consent is a fallacy. They are desperate for money and will do what they need to get it.
Others are controlled by abusive and violent "boyfriends" and so again meaningful consent is a fallacy.
The incidence of ongoing child sexual abuse and mental health problems amongst prostitutes is extremely high and thus it si questionable whether they can meaningful consent.
There may be a tiny number of women a la Belle De Jour who can meaningfully consent, but in general having sex with a prostitute is raping her. And many ex prostitutes will say that they experienced each act of sex as a pay per rape, even if they nominally consented at the time.