I think it's also a matter of skill and employer density, so to speak? There are some exceptions, but by and large - if you're in a typical metro area and want an IT job, you probably don't need to move. If you want a hospital job, you don't need to move. If you want a construction job, you don't need to move. You can, for some incremental gains, but it's usually not a necessity.

In the early to middle 20th century, you probably didn't have that. Most people lived in rural areas and you needed to move to get that job at Acme Steel Co.