If the world’s population were built at Brooklyn density, it would occupy about 70% of the state of Texas. At Manhattan densities, you could fit the whole world into Virginia and North Carolina. Leaving the rest of the world empty.
In practice, humans have to spread out in order to cultivate land to feed themselves, and agriculture aside, there are significant gains to distribution of population around the world (including the utility gains of satisfaction of varying tastes). But, you know, think about it.