Because a lot of modern dating implies the act of pre-marital intercourse. There would be in theory nothing wrong with muslims dating kafir if the pair bond/courtship remained pure and chaste, but such is the extent of the americanised/MTV dating culture these days, that prospect remains highly unlikely. We also aren't really that keen on associating with those that indulge in all the usual filth, alcohol, gambling etc but if someone made the effort to convert they would certainly be welcomed with open arms by the partners family.