Copyright is an artificial, government given Monopoly.
Market Mechanisms don’t work when faced with a Monopoly or work badly in situations distorted by the presence of a Monopoly (which is more this case, since Stack Overflow has a monopoly in the reproduction of each post in that website but the same user could post the same answer elsewhere thus creating an equivalent work).
Pretty much in every situation where Intellectual Property is involved you see the market failing miserably: just notice the current situation with streaming services which would be completelly different if there was no copyright and hence no possibility of exclusivity of distribution of any titles (and hence streaming services would have to compete in terms of quality of service).
The idea that the Free Market is something that works everywhere (or even in most cases) is Politically-driven Magic thinking, not Economics.