and then there's Clutch Cargo

Animation in the 30s was all intended to be a relatively standalone and intensive endeavor. There were no places devoted to putting out constant new content that was meant to be viewed weekly, and the shorts of then are on a whole much shorter than modern cartoon shows. Animation of that level takes a long ass time and a lot of people, and since animation these days also requires heavy duty story and voice work it makes sense that animation would be more simple to make room for them.