Civilization has gotten more complex in some ways. But the modern world shields us from a lot of other complexity: how many poisonous plants and dangerous animals do you need to identify? How many social/religious rituals do you need to perform completely perfectly lest you be ostracized from society?

One of the limiting factors on early-stage civilizations is that it takes pretty much a lifetime to transmit information from one generation to the next; between books, specialization, urbanization, and price signals (which are basically an API on human desires and the resources necessary to fulfill them), we need far less general knowledge than we used to.