Who would want to cite a random unreviewed preprint?
If you're writing a paper about a longstanding math problem and the solution gets published on 4chan, you still need to cite it.
Pure math tends to be much more conservative in citations than other fields though, and even when writing a paper about a longstanding math problem you wouldn't necessarily bother to include existing solutions. You reference the things you actually used, and even then you assume some common background knowledge for your audience and don't reference every little undergrad topology theorem or whatever. The point is to be honest with the reader about what was helpful for this work in particular, both to properly attribute things you actually used and to make any searches based on your work more targeted and fruitful.
That something is unreviewed does not mean that it is bad or useless.
Both models are fallible, which is why discernment is so important.