Or there's an increase in new buildings, or the number of buildings needing repairs increase over time, etc... With trades the number of self-employed or owners

Or there's an increase in new buildings, or the number of buildings needing repairs increase over time, etc... With trades the number of self-employed or owners is quite high - you can either do Job A by yourself for $X for company Foo, or be self-employed and do Job A by yourself for $X++. Same number of jobs exist, just less monopolized by larger companies. I don't know why, but this is what I found in the south-west of England when I lived there. I think the experienced people tend to start those orgs to charge more then get older and need to hire apprentices to do the work for them, and so the cycle continues.