Professions where you are paid by the hour are not scalable. A prostitute who charges $100 an hour only has 24 hours in a day. At some point, she will hit a ceiling on her earnings. Similarly, dentists, lawyers, contractors, bakers, and consultants can see only so many clients at a time.
By contrast, scalable professions allow you to make more money without an equivalent increase in labor / time. An author writes a book one time and his effort is the (basically) the same whether he sells 500 or 500,000 copies. A Hollywood actress need not show up at every screening of her movie to make money off it.
Read the article: Scalable vs. Non-Scalable Careers