There's a lot of relatively recent migration trends that spread things out.

The first is the move to the suburbs in the 60s, white flight and more common place car ownership made this happen, and our population was small and spread out enough that it didn't cause a lot of problems immediately with traffic and insufficient housing.

The second is the move away from small towns and rural areas. Many places, particularly midsize cities and rural areas in the Midwest didn't diversify their economy from manufacturing, so when manufacturing stopped being a major employer, people or often just people's children left. You can't go to an industry conference in my hometown without half the people you meet coming from Ohio, central Pennsylvania, Indiana or Missouri.

The third is air conditioning. The reason that Southerners were stereotyped as synonymous with rural bumpkins was for a while that was true, the only reason to be in the south was agriculture and resource extraction. Before air conditioning was commonplace, populations and industry was concentrated in the northeast and eastern Midwest. Imagine a triangle between Chicago, Richmond, and Boston. Cities like Atlanta and Dallas used to be comparable in population to Jersey City or Allentown because the population was so low. Even California was relatively sparsely populated and cheap compared to east coast cities.

This has accelerated recently as people continues to build housing and move in previously uninhabited or under inhabited areas like major cities in Texas, Florida, or Arizona. Most people in these places either moved there themselves or their parents did. So the way everyone speaks often averages out to generic "American".