Because facebook doesn’t generate the content. You must generate content in order to be a publisher. And once labeled as a publisher, you become liable for your speech. Generally those platforms allowing the posting of user generated content do not want to be liable for the speech, so they try their hardest to not be labeled as a publisher. This article explains in more detail the differences:

https://reason.com/volokh/2020/05/28/47-u-s-c-§-230-and-the-...