Well, people think that because doctors are in a healing profession that requires them to take an oath. Ethical issues are involved for doctors. Plumbers, not so much.
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.