Why would anyone build houses so far below the street level?
They didn’t! Explanation below.
When these house were built on Corlear Avenue in the Kingsbridge neighborhood of the Bronx, about 1900, Tibbett Brook still flowed, where Tibbett Avenue now runs. At that time, the brook was very low-lying, practically at sea level, joining the Harlem river a few hundred yards to the south.
Clearly, from these pictures, when the city filled in Tibbett Brook, they raised the street level up many feet. I imagine anguished meetings between the homeowners and the city, concluding with the authorities saying, “The street is going to be X feet high. You can keep your house, but you’ll have to figure out how to get up to the street on your own.”