Because early agriculture was all about planting things in flood plains... If you live on a flood plain you get automatic irrigation, all the nutrients in the

Because early agriculture was all about planting things in flood plains... If you live on a flood plain you get automatic irrigation, all the nutrients in the soil get replenished every year, you get some fish, etc... Most ancient civilizations (and certainly the ones that became the most populous) were built on flood plains. It took quite a few technological advances (Rome's aqueducts for example) and new agriculture techniques before civilizations thrived in other locations.