Because agricultural modernization universally requires at least an order of magnitude less labor, and society is then better served by those workers being in urban centers where they can work first in factories then transition to a service economy for the ones who'll work unskilled labor, and concentrate access to quality education for the ones that can contribute intellectually.

Those "ghost cities" that the media pearl clutched for a while there have for the most part been slowly filled in with previously agrarian workers making the urban transition.