It is just the way its made. A fairly heavy sheet brass wrap around for the walls, that would defy putting in rivet impressions. It was a lower cost kit at the time and the lack of rivet detail was largely overlooked. Other, more expensive kits of the day with brass sides having rivet impressions used thinner material or they used aluminum castings with details molded in. All Nation had them for its larger locomotive kits.
On my All Nation 4-4-2, I made a thin shim brass sheet of rivet impressions to apply to the smooth brass sides. It was labor intensive to be sure. I wanted to put rivet detail on cab as well, but after working on the tender, I though otherwise.
If I were to do this now, I would apply primer to the smooth brass tender shell and apply rivet decals, following the prototype pattern from photos. Rivet decals will bond better to primer or paint than they will on plain brass.
S. Islander