Because any situation where a man (whom is probably in his 30’s or 40’s) is hiring an underage woman and no one else will be around strikes me as creepy, that’s all. Of course there will be patienets, but before, and between, and he might ask her to work after hours on paperwork. I know someone personally who was in such a situation and was actually drugged using drugs a dentist uses (she was working for a dentist). It didn’t end well at all.
There’s dan likaf zechut, and then there’s being sensibly safe. Unfortunately women have to always make sure that they are safe from predators.
Unfortunately there’s a lot of bad people in this world.