Haven't heard plumbers launching side plumbing projects. But who knows, perhaps they do and just don't post on HN.

Many plumbers start their own business, or work on contract. As a startup founder, I gained a huge respect for anyone, in any industry that managed to create a business where people pay them money. From Mom&Pop shops, to plumbers, to tech founders. Most of the real problems are the same.

Plumbers typically work on a variety of different buildings