But an on-package memory bus has some of the advantages of HBM, just to a lesser extent, so it's arguably comparable as an "intermediate stage" between RAM chips and HBM. Distances are shorter (so voltage drop and capacitance are lower, so can be driven at lower power), routing is more complex but can be worked around by more layers, which increases cost but on a significantly smaller area than required for dimms, and the dimms connections themselves can hurt performance (reflection from poor contacts, optional termination makes things more complex, and the expectations of mix-and-match for dimm vendors and products likely reduce fine tuning possibilities).

There's pretty much a direct opposite scaling between flexibility and performance - dimms > soldered ram > on-package ram > die-interconnects.