I agree - that’s what people think.
Ethical standards apply to all aspects of life, regardless of profession.
In fact, one could argue that high ethical standards come more easily to those with humility - and that humility comes more easily to those of humble professions than the lofty ones.
An oath means nothing or everything, depending on one’s degree of ethical standards.
To think a doctor is necessarily more altruistic than a plumber because of an oath - is just silly.
But you are right - many people do think that.
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.