> Apple has managed to provide some fairly wide options in the M series though, so what are they doing that the others are not (aside from that they are effectively using more but smaller channels)?

They aren't using sockets. They are soldering it directly to the board. I figure that consumers aren't going to a buy a motherboard with a CPU and RAM soldered onto it. I could be wrong about that, but I wouldn't buy it.

They are also much more expensive. It's not that it can't be done since it is done with servers already -- it is that it can't be done for the price consumers want.