Why not? They already do the equivalent for SRAM. The big cost of a wider memory bus is routing it through the socket and the system board, which you're not doing since only the narrower bus goes through there. The wider bus is solely within the APU package.
You could also take advantage of the additional channels -- have e.g. 8 memory channels within the package and two more routed through the socket, for a total of 10. Now if you have 8GB on the package and 16GB off of it, you have 10GB striped across all 10 channels and another 14GB striped across two.
Continuing to have memory slots also allows you to sell chips with and without on-package DRAM and use the same socket. High bandwidth memory only makes much sense if you have a strong iGPU, since CPUs are rarely memory bandwidth bound. But low end systems and high end systems wouldn't have that, only mid-tier ones would. The low end system has a small iGPU where HBM is both unnecessary and too expensive. The high end system has the same small iGPU, or none at all, because it uses a big discrete GPU.
HBM is also more expensive than ordinary memory, but Windows will sit there eating several GB of RAM while doing nothing, so having some HBM and some DDR should lower costs compared with having the combined total in HBM.