There is no scientific evidence to prove this phenomena, but a possible answer could be that they are not scared of the cross and this myth has been made up by vampires themselves so brave people can run into their hives with a cross and without fear. Easy dinner is a good dinner.
You kind of got it but we dont eat people. We've adapted to society.
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Technically, vampires may not exist. But if they do, (and I know this since I did an extensive research project) not all vampires should be scared of the cross. The ones that are though, are scared because they are supposed to be unholy, dark creatures.