Don't sex workers use protection and therefore avoid all diseases?

You mean like herpes, molluscum, HPV, vulvodynia, and any of the other STIs that can spread through and around condoms and are well-nigh incurable? The truth is that many sex workers negotiate protection according to what they are comfortable with and how much money they are trying to get from an individual client, while many pornstars aren't allowed to use condoms at all. Just because you had sex with an escort who looked young and healthy to you and used a condom with you does not mean that she isn't secretly dropping heroin into her eyeball between appointments in which she engages in unprotected sex to raise extra drug cash on nights when you don't see her.

Also, condoms sometimes break. Even if the odds are very small when condoms are used correctly, people who get paid to have sex for a living have a lot of sex. Things which have a tiny chance of occurring every time someone has sex become much more of a risk when she has sex five times a day for a year, with hundreds if not thousands of partners over the course of a relatively short time.