Because s.230 expressly provides for them not to be treated as one. The worry at the time was that information services making editorial decisions (taking down

Because s.230 expressly provides for them not to be treated as one. The worry at the time was that information services making editorial decisions (taking down harmful content, in particular) would be treated as publishers, and so liable for what was left up. That creates an obvious moral hazard problem, encouraging bulletin boards and web hosts to refuse to even look at what's being posted, to avoid liability. So s.230 was added to the Communications Decency Act to make clear that the legal responsibility would fall only on those originally providing the information. This situation isn't mirrored outside the US, FWIW. IIRC England & Wales will impose liability for libels etc., but only if the host had actual or constructive knowledge of the content of the post and chose to let it stay up. That introduces quite a lot of legal uncertainty and a bias towards deleting controversial material but may be better overall. I don't really know.