I asked why humanity took so long to do anything at the start, and the Internet gave me its thoughts. Here is my expanded list of hypotheses, summarizing from comments on the post, here, and here.

Inventing is harder than it looks

Inventions are usually more ingenious than they seem. Relatedly, reality has a lot of detail.
There are lots of apparent paths: without hindsight, you have to waste a lot of time on dead ends.
People are not as inventive as they imagine. For instance, I haven’t actually invented anything – why do I even imagine I could invent rope?
Posing the question is a large part of the work. If you have never seen rope, it actually doesn’t occur to you that rope would come in handy, or to ask yourself how to make some.
Animals (including humans) mostly think by intuitively recognizing over time what is promising and not among affordances they have, and reading what common observations imply. New affordances generally only appear by some outside force e.g. accidentally. To invent a thing, you have to somehow have an affordance to make it even though you have never seen it. And in retrospect it seems so obvious because now you do have the affordance.

People fifty thousand years ago were not really behaviorally modern

People’s brains were actually biologically less functional fifty thousand years ago.
Having concepts in general is a big deal. You need a foundation of knowledge and mental models to come up with more of them.
We lacked a small number of unimaginably basic concepts that it is hard to even imagine not having now. For instance ‘abstraction’, or ‘changing the world around you to make it better’.
Having external thinking tools is a big deal. Modern ‘human intelligence’ relies a lot on things like writing and collected data, that aren’t in anyone’s brain.
The entire mental landscapes of early people was very different, as Julian Jaynes suggests. In particular, they lacked self awareness and the ability to have original thought rather than just repeating whatever they usually repeat.

Prerequisites

Often A isn’t useful without B, and B isn’t useful without A. For instance, A is chariots and B is roads.
A isn’t useful without lots of other things, which don’t depend on A, but take longer to accrue than you imagine.
Lots of ways to solve problems don’t lead to great things in the long run. ‘Crude hacks’ get you most of the way there, reducing the value of g

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deej