That is a good point, although perhaps not entirely true.
Getting licensed often requires either passing a test, or completing an internship, etc. Licensed professionals are also required to follow the rules (code, etc.). Well technically everyone is required to follow the rules, but if you get caught violating the rules, they can't revoke your license if you don't have one.
On the other hand, some areas are more 'progressive', shall we say, about allowing 'alternative' practices.
Arizona, for example, has some of the best rules for grey water. Basically as long as you don't produce more than 400 gallons a day, and follow some simple (and generally good) rules, you don't need to file for a permit or get any inspections, etc.
In Arizona, all new housing is now required to have grey water stub outs. They don't have to have a full grey water system, so they can (and often do) connect the grey water lines back into the sewer lines, but this has to be done OUTSIDE the house where they are accessible after construction is completed. This makes it much easier to install a grey water system in the future if desired. I really wish my house had been built after these rules went into effect, tunneling under the pad to access the drain pipes is a royal pain.
Rain water tanks up to 5,000 capacity that sit on grade, no permit required, etc.
In the county I live in alternative housing is fairly common (cave homes, geodesic domes,
straw bale houses, etc.) and if you have at least 4 acres you are allowed to build almost anything you want WITHOUT having any inspections. You still have to file for a permit and get your plans reviewed, but you don't have to have anything inspected.
Homeowners here in Arizona are allowed to DIY anything in their home, including electrical and plumbing work, although they are required to follow the code. Major projects require a permit and inspections (unless you foll under the 4+ acre exemption)
For any work on your home that you do yourself, and that is valued less than $1,000, you don't even have to file for a permit or get it inspected; although you are still required to follow the appropriate code. I frequently take advantage of that loophole by breaking major projects down into smaller, discrete, projects.