Because the guys at the shop are in a weird situation.

On the one hand, they know jack about you and appearances are all they have to go on. Let's be realistic here, human beings are programmed to make judgements about other people based on appearances.

On the other hand, they are constantly getting flak in the cycling press for focusing on selling nothing but carbon-fiber race bikes when all the person really wants is something for riding down to the store.

Obviously, the best policy is to spend some time chatting with a prospective customer and finding out where they are and what they want.

This doesn't just happen to women either. I've hand salespeople confidently tell me there is no way I'll be able to haul my body and my loaded touring bike over mountain passes with the gearing I had mere days after having done precisely that.