Why is dating haram in Islam when a lot of Muslims date?

Hi there, Islam says that it is haram for intimacy before marriage. Opposite genders can mix but not have relationships before marriage. Islam advises that even if not having a relationship that a boy and girl who are eligible for each other - i.e (not the girls father or brother) should never be alone in a room as when they are there is a third person in the room - The shiatan (devil). Who whispers to them and leads them astray into relationship outside of marriage even if that was not there intention. As humans are weak and sucb to temptations.