There is no one MLS, it isn't a cohesive database, it is made up of a ton of tiny independent services with fairly obnoxious policies and inconsistent data form

There is no one MLS, it isn't a cohesive database, it is made up of a ton of tiny independent services with fairly obnoxious policies and inconsistent data formats. Zillow gets their data generally from the various MLS around the country and aggregate data with public records