Could it be due to diminishing returns, as we pick the low-hanging/highest value fruit earlier?
Exploration / exploitation. After decades of asking "why?" you should have a decent model of the world, so that you can finally start acting on it. (If you keep asking "why?" forever, you become a smart procrastinator.)
Also, sometimes people are lying to you, so it makes sense to refuse further learning after you already believe you got your model generally right. Or some religious people may keep wasting your time by giving you more and more books to read... so there are just three possible outcomes: giving up, playing the game forever and wasting your time, or refusing to further investigate.
The specific trade-offs probably depend on IQ and general education (having good priors). Like, if you are stupid, you will get less benefits from keeping asking "why?".
I think a pattern that makes sense to me is cycles of exploration and exploitation: learn about the world, act on that understanding, use the observations you acquired from acting to guide further learning, etc. The world is big and complicated enough that I think you don't hit anything close to diminishing marginal returns on asking "why?" (my experience, if anything, has been increasing marginal returns as I've gotten better at learning things), although I agree that it's important to get some acting going on in there too.
Is it a natural phenomenon of human development, because you must be certain of your arguments if you don't want to perish in front of your opponents? Or is it the consequence of a wrong educational system, which trumps their curiosity saying "stop asking questions, learn what I tell you, unconditionally" and penalizes them when the do not brag parade enough what they have memorized?
There are other possibilities. To name a few hypotheses:
My sense is that parents and people who study education already have ideas about what leads children to be more or less curious, but that there are practical obstacles to implementing many of their ideas. For example, having a mentor who can answer questions is good for creativity--but people who can play that role well are scarce.
It's because their curiosity starts getting worse results, when their questions pass the point where available adults can answer easily, before [my emphasis] they become literate enough to pursue curiosity independently.
This is quite an interesting mechanic and it seems to be related to Dr. Jamchie's comment on discovering other pleasures and motivators as well. If assumed to be true, it leads to make the experimental prediction that, if a kid develops in an environment rich in adults who can answer their questions up to a higher level of difficulty, the kid is likely to grow up into a pretty science-and-curiosity-oriented adult... which does sound quite obvious, actually.
For me, personally, it was some combination of discovering other, at that time, stronger pleasures and motivators than pure bliss of chasing the truth; depleting most of available resources of interesting information and feeling already smart enough for all intents and purposes. Although curiosity have found it's way back to my life several times since.
I've recently written a post in my own blog, asking a question: unfortunately it's in Italian, so I'll try to summarize it in English here.
Kids are born scientists: they are curious, ask questions, are eager to seek an understanding on the world around them; but then they turn into adults: most of them stop asking questions, even to themselves, stop being permeable to new knowledge, and appear to have granitic certainties which no experiment could ever shatter. Those who later turn to become scientists, will have to learn again the virtue of humble curiosity.
I wonder why. Is it a natural phenomenon of human development, because you must be certain of your arguments if you don't want to perish in front of your opponents? Or is it the consequence of a wrong educational system, which trumps their curiosity saying "stop asking questions, learn what I tell you, unconditionally" and penalizes them when the do not brag parade enough what they have memorized?
I guess both are true, but my question is: to what extent is it a biological fact, and to what extent is it a cultural one? To what extent should we fight it teaching people evolutionary biology facts and biases, and to what extent should we fight it changing the educational system in ?