To compete with China's and India's labour costs. To hold on to economic leadership. Making yourself irrelevant on the world economic stage will impoverish a nation, relative to other nations. There's moral panic of the western Capital in that two nations with 1.5 billion population each will eventually concentrate all economic activity, because their population levels are too exaggerated and material costs are very low compared to everyone else.
I do not think simply raising population will do the trick. One has to compete with socialist mode of production and the resulting centralisation of resources achieved through this mode of production, which yields superior labour/material costs.
In other words, the only way to defeat the Chinese long terms is not through trying to catch up population wise, but by adjusting the mode of production and by becoming self sufficient in critical industries. Besides, you won't talk the western population into having way more babies. Nor will the citizens en masse agree to have hordes of foreigners immigrate and compete with them.