I noticed that an answer was deleted from a question that I commented on (Why would a flight from North America to Asia sometimes fly over the Atlantic?, the deleted answer had some reference to Riemann geometry).
On Math SE (which I am more familiar with) many answers get deleted, but the answer (and usually most of the non offensive comments) remain (for example, https://math.stackexchange.com/a/2871309/27978).
Why is this? It would seem useful to see deleted answers (unless they contain objectionable content, of course).