Why are Mesopotamia and Egypt called different civilizations?

Because they have a different culture, customs, habitat, and so on. That is, there are many differences between them. Ancient Egypt was a state ruled by a pharaoh, he was the sole ruler. At the same time, there was no single state on the territory of Mesopotamia for a long time. There were city-states ruled by priests. The life of the ancient Egyptian civilization was very strongly connected with the Nile River, and in Mesopotamia there was no such strong dependence.