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.
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.