Early civilizations did not appear out of nothing 6000 years ago. The earliest evidence of cereal harvesting in the Middle East is from 23000 years ago. The transition from hunting and gathering to sedentary farming communities took over 10000 years, and those communities needed thousands of years to grow into large sophisticated cities.

It's also good to remember that there has been an ice age for the last 2.5 million years. The climate is generally too cold and dry for agriculture, except during relatively short interglacial periods. Maybe there was an opportunity for a civilization to develop in the Eemian period 130k to 115k years ago, but that was likely the only window of opportunity.