Preprints are great & everyone should post a preprint as soon as they're ready to share what they've been working on. That said, there's a big difference between journals and preprints, not just in the production quality or the improvements made after review, but in the whole hidden infrastructure that supports the discovery of artcles, the indexing, preservation, linking, etc.
It's worth pointing out that running a preprint server isn't exactly cheap, either: https://www.sciencemag.org/news/2017/04/biorxiv-preprint-ser...