Well, in the case of doctors, they take an oath and are supposed to be bound by it. Hippocratic oath—Do no harm. So, to do otherwise would mean doctors are violating that oath as well as being self-serving and greedy along with being dishonest—morally lacking in many ways. Much worse than a plumber who does you wrong.
If the fact that doctors take an oath makes you trust the entire profession as a whole - good luck to you - you’ll need it.
It is buyer beware in all transactions - do your homework.
There are good people and bad people in any profession - competent and incompetent - altruistic and selfish.
You need to judge skill and character by the individual - it makes no sense to rely on the reputation of an entire profession - just because they take an oath or earned a degree.
Yes a lot of people do just that - but not the smart ones.