That's kind of like saying no one should go to work because some people are illegal immigrants.

Having all prostitutes have to show their birth certificate, background check, address, phone number, etc. Ensures that no one underage is doing it and helps police find missing people faster. If there is a known brothel, it can be audited like hospitals to make sure they are keeping records and account for all of their transactions and staff. They should be able to prove that the missing person was not at their facility fairly easily.

It also keeps prostitutes safe because they can be tested for STDs and as well as the customers. Anyone who abuses a prostitute can be arrested.

There are a ton of reasons why both prostitutes and customers are safer when it is legal.