We have the postal system and telephone system, which are in theory (and I think most of us believe) content-neutral. You can say whatever you want over these channels, and, as far as we know, the USPS and phone company don't investigate the content and block naughty thoughts, nor are they held liable if we say naughty things or conspire to commit a crime over their channels.
Newspapers, magazines, and TV are at the other end: If they publish naughty stuff, they're going to be held accountable, and therefore they exercise editorial moderation and selection over what they publish.
Social Media and Internet forums are in this weird separate bucket that was simply conjured up by Section 230. They get to have their cake and eat it too. They can both 1. editorialize and moderate their users' content but 2. dodge liability over what they publish. What a great deal!
I think whether you are liable for what your users post -should- come down to whether or not you editorialize and put your thumb on the scale of what gets posted and shown. If you're truly a "dumb pipe" that allows everything, then you should not be liable for what your users send over the dumb pipe. But the minute you exercise any moderation or curation, you are effectively endorsing what you are publishing, and should share liability over it.