There are plenty of neighborhoods in the Los Angeles metro with these characteristics. It's just that many of them are among the more expensive areas (Downtown

There are plenty of neighborhoods in the Los Angeles metro with these characteristics. It's just that many of them are among the more expensive areas (Downtown, Koreatown, Santa Monica, Fairfax, Echo Park, …). "City population density" doesn't strike me as a good metric for judging a city's ability to support good public transit. It's not like Los Angeles (as a whole) was any denser when it boasted an extensive network of streetcars and electric rail spanning the city (and surrounding towns like Pasadena and Santa Monica); LA city population was ~1.5M in 1940 and is ~3.97M today with no substantial change in land area.