Why were egypt's pharaohs unusually powerful rulers

The concept of separation between religion and state wasn't valued in Ancient Egypt. Therefore, the two were heavily interconnected. 
One of the traditions of the religion of the Ancient Egyptians was that the Pharaoh was the living embodiment of the god Horus, who was believed to be a god of birthright, among other things. After the Pharaoh's death, he/she was identified with the god of the Underworld, Osiris. Thus, the living ruler was divine, and his word was the word of the gods.