Because we would usually expect employers to compete for workers up to the wage-level of marginal productivity. If you're underpaying your workers and we both have the same technology, I can lure them away by paying them more, until they start demanding wages higher than our tech can allow. To compete with me, you then have to raise wages too, and the market evens out at marginal wages equaling marginal productivity.

One of the Big Questions not covered by the article is: how competitive have labor markets been over time?