Because of professionalism. The root of term used for “professions” like law, medicine, engineering etc. is that one professes to a code of ethics. That code should overrule base desires.
We generally wouldn’t accept a doctor who views and treats patients primarily as an income stream despite greed being a near-universal human drive and we shouldn’t expect a professor to view subordinates as potential romantic partners. Acknowledging the drive exists isn’t a reason to condone it.