the absence of a developed economics until the mid-18th century remains a startling anomaly in the history of ideas. Why was that?
I would note that the theory of probability also was developed surprisingly late. I think this is because our mathematical intuition is about material things, and we conceive of a thing as being either there or not there. We do not think in terms of a thing as being there with probability .75.
In economics, perhaps Adam Smith was wrong in claiming a universal propensity to truck and barter. A universal propensity to engage in exchanges with friends and family. In Alan Page Fiske's terminology, there is a universal propensity for communal sharing, for equality matching, and for hierarchical control. But trading among strangers in competitive markets is not so universal.
My view is that historically there was a universal propensity for plunder and coercion. It could be that only in the late stages of the British empire, around the time that Adam Smith was writing, that people really began to be accustomed to market economic activity.
In that case, it would not be surprising that economics itself developed late. Conversely, the fact that there was no Greek or Roman Adam Smith is consistent with my view that the Greeks and the Romans did not really have modern market economies.