In math they often still skip those motivation, related work overview stuff and just do some math.

But I actually do think it's good to force people to think about the "why" question a lot.

Maybe! But also maybe niche research can just be for the niche researchers. The work is funded, and therefore the "why" hoops have been jumped through, right?

And anyway, aren't reviewers themselves supposed to be able to connect dots?

When I was an active reviewer I tried not to ever say "Who cares" in so many words and just focus on the technical contributions. There's enough overinflation/fraud in papers that we dont need to be encouraging it by asking them to over-over inflate a possible reason for the work that is beyond the obvious ones that reviewers in the same field would appreciate.

All IMHO. There's no "right", just "preferred".