There is a big difference between painting your own house as a hobbyist and painting other houses as a professional. Professionally painting several houses at once is an entirely new skill set. Skills like:

• planning exactly where you paint so you don't interrupt flooring and other projects.

• painting in a manner that is time efficient but does not miss anything.

• remembering to bring and put down drop sheets everywhere.

• coming with the exact amount of paint you need for the day so you don't have to pick some up during your shift.

• being meticulous about taping off areas while still keeping time.

• will not leave a mess with their equipment when they are done.

This is by no means an exhaustive list; I'm not a painter. I have however experienced both kinds of painters.

You may be able to do everything right on somebody's house without training, but it'll take you weeks. Somebody trained to do this will take much less time, can do several houses at once and can keep the quality perfect consistently. Not an easy task.