It's a combination of "the ads subsidize the production of the content somewhat i.e. you aren't paying the full price" and "everyone is desensitized to ads now

It's a combination of "the ads subsidize the production of the content somewhat i.e. you aren't paying the full price" and "everyone is desensitized to ads now anyway so we might at well show them in every place we can possibly get away with it". FWIW I think it's mostly the latter (e: was thinking specifically of cinema when I wrote this - nl is right it depends on the medium).