There's no "cannot", it's a conscious choice - if you have more space than necessary that absolutely does not require that you expand to fill all of it. And in particular this has caused issues because American cities tend towards sprawl, filling up area with low-density housing that requires significantly more infrastructure per capita, and implementing zoning laws to prevent higher-density development even as the population grows, resulting in housing shortages.

Not Just Bikes has some excellent content about this, e.g.: https://youtu.be/XfQUOHlAocY