This has actually been the law in i.e. Germany and I think many other countries as well. As long as the platform doesn't interfere with the content (§ 8 Abs. 1

This has actually been the law in i.e. Germany and I think many other countries as well. As long as the platform doesn't interfere with the content (§ 8 Abs. 1 TMG) they are directly accountable for the content (Which doesn't mean that they don't have to take it down in case of an infringement for example). But as soon as they moderate said content they are legally liable. And this is exactly what social media platforms have done and that is one reason why they are responsible for their content now. This is of course only one part of the truth, because at the same time there has been a shift in perception of these platforms and other laws (i.e. Netzwerkdurchsetzungsgesetz) have been specifically aimed at social media platforms to hold them accountable.