This question concerns only proven statements. I don't know if research papers do, but most math textbooks don't. Counterarguments:
Space?
1.1. The increased length from explaining the discovery is justified; if proofs' discoveries aren't explained and revealed, how can they be learned and discovered, especially to solve unsolved questions?
1.2. The discovery may be more vital than the proof: easier is deducing the latter from the former, rather than searching the latter for the former. So if space is a problem, why not focus on the discovery and leave the proof to the reader, instead of vice versa that is the status quo?Ignorance of the discovery? Maybe these flashes of genius cannot be described? This seems unlikely for statements proven long ago.
Desire for students to toil, and painstakingly discover the discovery themselves? But this is inefficient; if certain theories expended centuries for a famous mathematician, how can a math student discover within a day?
Suppression of knowledge? This is least likely, and paranoid; maybe authors don't wish students to learn the discoveries easily, because then future students would outperform them?
E.g. Cauchy-Schwarz Inequality is often proven without its discovery explained. Textbooks don't explain how a student would divine $ \bf{0} \le \|\bf{u} - \cfrac{\langle u, v \rangle}{\|v\|^2}\bf{v}\|^2$.