Can I do my own plumbing if I do it according to the codes and I am not in a union? Also why do I have to pay for a plumber that is qualified in drainlaying, gasfitting and roofing when all I need is to fix a small leak in my faucet? It's like when you need to reinstall Windows on your PC you cannot do it yourself and have to get a software engineer for $500/hr who can write assembly code and knows the difference between merge and quicksort.
Many plumbers are also qualified in drainlaying, gasfitting and roofing as well. If you work with gas then lives depend on the quality of your work.
You also have to be physically fit, willing to get up early and sometimes deal with really unpleasant situations (e.g. wade through shit).
"Average" plumbers in AU might only get 60-80k (seriously, check the ads), which is actually not that great an ROI on the investment.
It's not until you specialise, become senior, go out on your own or start doing lots of overtime that you make money.