Why did the Greeks have many gods?

We are used to hearing that the times of Antiquity are the cultural and technical rise of the Ancient World. Yes, we cannot deny the fact of progress, but we also cannot idealize it.
The Greeks had many Gods, because they were still at the stage of polytheistic religion – a typical sign of paganism. Monotheism, we know Christianity, Islam, Judaism – all these monotheistic religions will arise later. Unfortunately, the pure Greek civilization did not live to see such changes.