Peter Gerdes says:

As the examples of the Nicaraguan deaf children left on their own to develop their own language demonstrates (as do other examples) we do create languages very very quickly in a social environment.

Creating conlangs is hard not because creating language is fundamentally hard but because we are bad at top down modelling of processes that are the result of a bunch of tiny modifications over time. The distinctive features of language require both that it be used frequently for practical purposes (this makes sure that the language has efficient shortcuts, jettisons clunky overengineered rules etc..) and that it be buffeted by the whims of many individuals with varying interests and focuses.

This is a good point, though it kind of equivocates on the meaning of "hard" (if we can't consciously do something, does that make it "hard" even if in some situations it would happen naturally?).

I don't know how much of this to credit to a "language instinct" that puts all the difficulty of language "under the hood", vs. inventing language not really being that hard once you have general-purpose reasoning. I'm sure real linguists have an answer to this. See also Tracy Canfield's comments (1, 2) on the specifics of sign languages and creoles.

The Secret Of Our Success described how human culture, especially tool-making ability, allowed us to lose some adaptations we no longer needed. One of those was strength; we are much weaker than the other great apes. Hackworth provides an intuitive demonstration of this: hairless chimpanzees are buff: