That's what HBM is actually. The memory dies are directly next to the GPU die, on the same substrate. The main difference between Apple SoC and GPUs is the former use regular LPDDR while GPUs use HBM.

One of the key points of HBM is that dies are stacked up with many, MANY, more signals and channels. That's how NVIDIA has a memory bandwidth an order of magnitude higher than M4: 550GB/s for the M4 Max, 4.6TB/s for H200. And yes, that's bytes per second, not bits per second.