> why so few of AMD's GPUs have official ROCm support

Because "official ROCm support" means "you can rely on AMD to make this work on your system for your critical needs". If you want "support" in the "you can goof around with this stuff on your own and don't care if there's any breakage" sense, ROCm "supports" a whole lot of AMD hardware. They should just introduce a new "experimental, unsupported" tier and make this official on their end.