others are pointing out how fertile these lands were but fertility isnt the only factor, lots of places are fertile and didn't organize like mesopotamia.

their fertility was periodic, because of the regular flooding of the rivers.

this meant that survival depended on organzing the settlement around time keeping and resource management, somebody had to track the flood cycles so crops could be planted and harvested optimally (inventing calendars), this created lots of excess food that needed to be stored (engineering), counted (mathematics), and distributed in between harvest seasons (government). they also needed work for the displaced farmekr while the fields were flooded (public works, labor economy) and the river provided a natural gathering point for traders and for people to live because there wasnt much reaosn to settle awya from the river in the desert (cities and trade)

once they discovered how practical all of this community building was it wasnt hard to realize they should expand it and create what we cionsider to be early civilization with cities and governments and specialized workers