Why did ancient Egypt civilization begin?

A difficult question. Consider the astronomical circles at Naba Playa, in Nubia, which some believe reflect stellar alignments as far back as 16000 to 15000 years ago.
But others interpret the same evidence as less than 10 000 years ago.
Some of the Egyptian Pharaohs may have come from this region.

As to the WHY, there is safety in numbers and a communal aggregation has so many advantages that must be regarded as inevitable.
The sharing of tasks, which in turn leads to specialization of tasks.
The economies of scale didn't have to wait for Adam Smith to appear.

Given the origin of Homo not too far south, this area with its rivers and plains would be a natural settling place.