"Because if it's possible to be more intelligent, why aren't we?"

Because deep abstract thoughts about the nature of the universe and elaborate deep thinking were maybe not as useful while we were chasing lions and buffaloes with a spear?

We just had to be smarter then them. Which included finding out that tools were great. Learning about the habits of the prey and optmize hunting success. Those who were smarter in that capacity had a greater chance of reproducing. Those who just exceeded in thinking likely did not lived that long.

Is it just dumb luck that we're able to create knowledge about black holes, quarks, and lots of things in between which presumably had zero evolutionary benefit before a handful of generations ago?

Basically yes it is luck, in the sense that evolution is just randomness with a filter of death applied, so whatever brains we happen to have are just luck.

The brains we did end up with are really bad at creating that sort of knowledge. Almost none of us can. But we’re good at communicating, coming up with simplified models of things, and seeing how ideas interact.

We’re not universe-understanders, we’re behavior modelers and concept explainers.

I wasn't referring the "luck" factor of evolution, which is of course always there. I was asking whether "luck" is the reason that the cognitive capabilities which presumably were selected for also came with cognitive capabilities that almost certainly were not selected for.

My guess is that it's not dumb luck, and that what we evolved is in fact general intelligence, and that this was an "easier" way to adapt to environmental pressure than to evolve a grab bag of specific (non-general) cognitive abilities. An implication of this claim would be that we are universe-understanders (or at least that we are biologically capable of that, given the right resources and culture).

In other words, it's roughly the same answer for the question "why do washing machines have Turing complete microcontrollers in them when they only need to do a very small number of computing tasks?" At scale, once you know how to implement general (i.e. Turing-complete and programmable) computers it tends to be simpler to use them than to create purpose-built computer hardware.

Evolution rewarded us for developing general intelligence. But with a very immediate practical focus and not too much specialisation.

I don't think the logic follows here. Nor does it match evidence.

The premise is ignorant of time. It is also ignorant of the fact that we know there's a lot of things we don't know. That's all before we consider other factors like if there are limits and physical barriers or many other things.

While I'm deeply and fundamentally skeptical of the recursive self-improvement/singularity hypothesis, I also don't really buy this.

There are some pretty obvious ways we could improve human cognition if we had the ability to reliably edit or augment it. Better storage & recall. Lower distractibility. More working memory capacity. Hell, even extra hands for writing on more blackboards or putting up more conspiracy theory strings at a time!

I suppose it might be possible that, given the fundamental design and structure of the human brain, none of these things can be improved any further without catastrophic side effects—but since the only "designer" of its structure is evolution, I think that's extremely unlikely.

Some of your suggestions, if you don't mind my saying, seem like only modest improvements — akin to Henry Ford's quote “If I had asked people what they wanted, they would have said a faster horse.”

To your point though, an electronic machine is a different host altogether with different strengths and weaknesses.

Well, twic's comment didn't say anything about revolutionary improvements, just "maybe we're as smart as we can be".