why does the development have to be furthered so much by children?
Because children don't have an existing structure overlaying whatever "natural" structure we may have hard-wired in our brains. One of Pinker's other notions (or at least a notion that I was introduced to by Pinker) is that grammar is reinvented every generation. We come into this world knowing not one iota of language yet, by the time we are 3 or 4 we can form semi-complex sentences and recognize "bad" grammar when we hear it. This grammar is formed from nothing but listening to and imitating the speech we hear about us; being rewarded for successful usage and mocked for unsuccessful usage. The few times we get explicit language usage instruction it is either wrong (i.e., not representative of the way our elders actually use the language, e.g., the can/may "rule") or a correction of application of a rule to an exception (e.g., a child who, up until now has been saying something like chiller as the plural of child "correcting" it to chiles or chillers).
By the time we become adults we have a good solid structure that we can use, even if that structure is almost non-existent as in the case of pidgins. The ability to use this innate grammar building talent seems to run out at about age 5 or so, which explains why many people continue to use the grammar they developed when first learning language despite the efforts of teachers to correct them. They just ain't getting them in time.
