That's how Chester Finn poses the following question in an editorial in the Wall Street Journal
Over the past half-century, the number of pupils in U.S. schools grew by about 50% while the number of teachers nearly tripled. Spending per student rose threefold, too. If the teaching force had simply kept pace with enrollments, school budgets had risen as they did, and nothing else changed, today's average teacher would earn nearly $100,000, plus generous benefits. We'd have a radically different view of the job and it would attract different sorts of people.
Yes, classes would be larger -- about what they were when I was in school. True, there'd be fewer specialists and supervisors. And we wouldn't have as many instructors for youngsters with 'special needs.' But teachers would earn twice what they do today (less than $50,000, on average) and talented college graduates would vie for the relatively few openings in those ranks.
Rule one in economics: People respond to incentives.