Very simple. Throughout the world, cities (urban areas) grow fast than the villages (rural areas) due to various reasons mainly due to the Government policies and the industrialists willing to invest only in urban areas than rural areas, thus creating more job opportunities in the bargain. It is because, industrialists and multinational companies expect quick return on the money invested.
Therefore, when urban areas quickly offer a lot of job opportunities, it is but quite natural that all educated men and woman flock to cities and other rural areas. As otherwise, having finished their education and obtained a degree too, what can they do sitting in villages? After all, it is necessary to earn something too, to sustain your life.