I think it's due to the heating issue on the boards themselves. HBM promises to make that issue a bit better since the memory chips are closer to the GPU which

I think it's due to the heating issue on the boards themselves. HBM promises to make that issue a bit better since the memory chips are closer to the GPU which makes it easier to cool off than before. But there's a limit due to HBM growing vertically. So, I doubt we'll see massive memory chips on consumer graphics cards in the near future.