To put it in a nutshell, we are engaging together in over an hour of close physical contact, breathing the same air with people who may be highly contagious and not know it.
You have never had an appointment with any doctor or ever had another medical procedure that creates similar exposure for either you or the practitioner.
This has always been true of manual therapy.
What is different now is that we know the factors that increase the spread of COVID are extended close contact, enclosed space, sharing the air in that space, and extended conversation or other forceful exhalations like coughing. These aerosol viral particles can hang in the air for hours without adequate filtration.