I guess that no one knows the mystery behind this. It seems a bit odd given that those are the 35mm print sizes.
It's because there is no standard anymore (the original paper sizes matched the original 5" x 4", 7" x 5" and 10" x 8" cameras very nicely!!!!!!)
Picture frames and mounts, and traditional darkroom printing papers have always been available in 5" x 7", 6" x 8", 8" x 10", 10" x 12", 12 x 16", 16" x 20". These did not suit 35mm (3 x 2 ratio) printing either, but also did not match 6cm x 6cm (square format 120 film), or 6cm x 7cm, 6cm x 9cm, etc etc etc, they were produced for plate film cameras.
The paper sizes suited the original formats, but there are now so many different formats available that the easiest option for paper manufacturers was to adopt universal standard paper sizes, hence we now get A4, A3, A3+, A2 etc. The reduction in cost associated with producing papers in a few standard sizes far outweighs the extra cost that would have been imposed on us by manufacturers having a huge range of sizes, and also it would not be economically viable, especially for the few remaining bricks and mortar retailers, to stock such a large range of papers.
The reason people don't understand paper sizing is that they think that 35mm (3 x 2 ratio) is the universal format, but the formats were developed a long time before 35mm.
HTH.