The actual reason the M1 Macs cap out at 16GB is that they use LPDDR4, and that on a 128-bit bus caps out at 16GB. Assuming that they move to LPDRR5 as it becomes available (hasn't Apple generally been pretty quick on the uptake on new RAM standards?), that limit goes up to 64GB.
It's a conservative spec for sure given the rest of the M1. Samsung has shipped their own LPDDR5 in the Galaxy S20 since February. Maybe production capacity is still too limited.
I believe that Samsung co-designed the memory with their interface block, and then it was effectively codified as the standard. Most other companies had to wait for the standard to be formed before they could start work on their IP. This gave Samsung a huge head start.
