Why can’t we say hello over the threshold?

It is generally considered rude to say hello to someone over a threshold. This is because it implies that the person is not welcome to enter the space. It is also considered to be a sign of disrespect, as it implies that the person is not worth the effort of stepping over the threshold to greet them. Furthermore, it can be seen as an invasion of privacy, as it implies that the person is not allowed to enter the space. Therefore, it is best to greet someone in a more appropriate manner, such as by stepping over the threshold and saying hello in person.