Why strengthening the U.S.-Mexican border leads to more illegal immigration

Peter Shrag has a good piece on illegal immigration.
Most efforts to secure the border result in the unintended consequence of more illegal residents. His bottom line?

The most promising workplace strategy, which has hardly been tried, would be far more rigorous enforcement of labor laws on wages, hours and overtime, and of worker safety laws. That would sharply reduce employer incentives to hire and exploit illegal immigrants.