Once boys' and men's challenges are clear the question 'why now' quickly becomes 'why didn't we see this sooner?' The answer? Virtually every society that survived did so by socializing its sons to be disposable.
At the root of the shy temperament is a deep fear of social judgment one so severe it can sometimes be crippling. Introverted people don't worry unduly about whether they'll be found wanting they just find too much socializing exhausting and would prefer either to be alone or in the company of a select few people.