I think it may have to do with versioning as well. If you put a paper leaflet into the box, you need to replace it or add an erratum in every box as soon as you realize you want to update / correct it in some way. It also makes it easier to offer multi-lingual instructions without having those awkward leaflets/booklets that seem very substantial, until you realize that there are only a few sentences in your particular language and the rest is in Malaysian, Swahili and Frisian. Finally, there's a (debatable) sustainability implication of adding more paper to the product - although of course this is kind of insignificant in relation to the box, the bottle, the contents and the shipping etc.