I live in a very walkable area. Partly because it’s walkable, it’s also desirable and thus expensive, and tends to attract people who are either wealthy or at the start of their upward mobility - young people from families with means who are fresh out of college and reporting $50k in annual income now, but will likely be reporting $150k in annual income in the next decade. This is the story of most larger northern cities, and it’s a pretty simple explanation for the phenomenon explored here.