There are some shady people in every profession, but most tradespeople aren’t ripping off customers too badly. It’s more like people who don’t work in the trades aren’t aware how expensive the operating costs are, like getting licensed/bonded/insured etc. Running a plumbing business is more financially complex than just buying materials and plugging them in to the clients’ home, so of course as a DIY homeowner you can skip a lot of nonsense. If you can do the work yourself, great! You save money and it gives the pros more time to help out folks who can’t do the work themselves.