The math equivalent of a negative result would be more like "I tried for months to prove this theorem, and it didn't work".

The difference is that in math "here is a proof" is all you need, while in sciences people are typically sharing what amounts to collections of observations. If you publish the collections that lead to interesting conclusions, but not the ones that are boring, people looking at what's been published are misled.

A negative result means simply they didn't find compelling evidence for their hypothesis. Nothing is proven or disproven (although it adds weight to the idea that the hypothesis is not true), so people think it is unexciting. For that and other reasons, the system incentives not publishing these, which is a pretty big problem.