This is an old and interesting question: how do we explain apparent trends in evolution without falling into the trap of teleology or goal-directed evolution, the idea that evolution is working toward a particular end point?
I don't know the answer, and I don't know if anyone knows the answer, but at least two books have tackled the question in a serious way.
... [f]illing a previously empty niche does wonders for the reproductive success of an organism; variations which increase size make new niches available, and so are favorably selected. But, owing to constraints imposed by basic physics and chemistry, larger organisms must be more specialized internally, i.e. more complex, to be as efficient as smaller ones, or even just to survive, so the selection is especially favorable for larger and more complex organisms. Because of the way developmental processes work, this complexity will probably be retained even by later, smaller organisms in other niches. Voila: the evolution of complexity, by means of natural selection.
Bonner's argument might or might not work at the level of cultural evolution.
- In contrast, McShea and Brandon argued in a 2010 book that random variation by itself should tend to increase complexity. McShea's summary:
even when forces and constraints are present, a tendency for complexity to increase is always present. The rationale is simply that when selection is absent, the parts of an organism should tend spontaneously to accumulate variation, and therefore to become more different from each other ... A consequence of the [Zero-Force Evolutionary Law ] is that we do not need natural selection to explain complex tissues and organ in organisms.