This link is very informative with regards to lead paint, it wasn't some corporate abuse, there were compelling reasons to use lead paint and hence it was accepted. It even notes that it was rarely found in interior paints by the forties, it persisted in outdoor paints because it could withstand the elements.

That strength was why it was used in the first place, you could wash it and the concerns of that age were communicable diseases that were slowed or prevented by washing, which the previous interior covering being wallpaper was not so durable and was suspected of easily harboring disease through pores and the glue.

Top it off with the US Government basically telling you to use it. Many of the health concerns for lead use were only seen as manufacturing dangers, end products were not considered unsafe.