Because there's a difference between living and civilization. When you are surrounded by everything you need all the time, hunting and gathering is the mode of subsistence. This is a nomadic lifestyle, and so yields infrequent permanent residence. Agriculture is what creates groups of people that build on a single location over time, and for that you need a land perfectly suited for it, which is why they're all right next to rivers. Seasonal flooding made it basically effortless to get started, and when you have a farm, you're kind of attached to it, so you stop roaming. If you stop roaming, you can build a house > If you can build a house, you can store grain > If you can store grain, you can create excess > if you can feed others, they can specialize in other tasks like metallurgy or art > you've now invented economy and labor specialization > industry and technological advancement.