It's a learned behavior sexworkers usually teach each other. If a whore just lies there naturally looking bored during routine fuckwork the customer is going to feel like fucking her really hard to try to get some reaction out of her - which is the last thing hookers typically want. Prostitutes want their customers to cum quickly and with minimal wear & tear to their bodies, hence acting like they are practically in pain from the size of the customer's penis and the vigor of their pumping action - to discourage them from fucking harder and get it over with faster. Heavy on the foreplay and theatrics to minimize the time spent rubbing latex against their battlehardened holes.
In japan they squeal and say "kimochi", in brazil they loudly breathe in through the mouth and say "ai gostoso". It becomes so ingrained that even the ones doing beast porn will still -by reflex- dirty talk the zoo animals they copulate with.
