[Question] Why does History assume equal national intelligence?

So… what I have often wondered when reading about historical events is that it is very rare to read a story where it is asserted that historical actor A was victorious over actor B due to actor B being a bit of a dumbass, or actor A being smarter.

In fact, one could be forgiven for thinking that this is some sort of taboo; the outcomes of wars might be down to various sins such as pride, ill-advised beliefs about ones superiority or ascribed to the decisive tactics used, but almost no action is ever being attributed directly to cleverness, or any loss to thoughtlessness or just being dumb. Now one might argue that, say, the tactics of an army are the outcomes of intelligent thought, and of course this is true to some extent. But I am just curious why it is that history books in general basically appear to work on the implicit assumption that each nation being discussed has the same level of intelligence and the whole concept has no mentionable role to play in the run of history and the events as they transpire.

And this is very odd indeed, because it would seem to me that the vast majority of outcomes of a given interaction between nations, rulers and populations depend to a critical degree on the intelligence of the parties involved. Yet, this aspect appears to be hardly ever discussed. You’d sooner read about the ill health of a ruler, or the kind of extravagant palaces he built… intellectual capacity might also be mentioned, though if so, often as a general (inferred) trait, without elaborating on its impact in actions taken.

So why is that? Perhaps it is too hard to measure or ascertain...? Easier to count the number of cannons than the “IQ” of the generals...? I’m curious what you will say :)