If I sell tins of beans, and some of them turn out to be poisonous, you might reasonably think that it is my responsibility to remove the poisonous ones. I could argue that I shift a lot of beans, too many to sort through, but I don’t think you’d buy that.
Online content is the same. The overhead is quite steep if you start from the assumption that it’s not really Facebook’s job to look after their own material. Which is exactly what Facebook et al would like you to believe. I have sympathy for the argument that a search engine is not responsible for other people’s content, but Facebook host content on their own site, for their own profit. It seems reasonable that they should put some effort into removing that content when it is illegal.
(Yet again I find myself wondering why the UK is so determined to leave the EU, when it does good stuff like this.)