LPDDR5 is "low power DDR5". The difference between that and ordinary DDR5 isn't that it's faster, it's that it runs at a lower voltage to save power in battery-operated devices. DDR5-6400 DIMMs were available for desktop systems around the same time as Apple. Servers are more conservative about timings for reliability reasons, the same as they use ECC memory and Apple doesn't. Moreover, while Apple was soldering their memory, Dell was shipping systems using CAMM with LPDDR5 that isn't soldered, and there are now systems from multiple vendors with CAMM2 and LPDDR5X.
Existing servers typically have 12 channels per socket, but they also have two DIMMs per channel, so you could double the number of channels per socket without taking up any more space for slots. You could also use CAMM which takes up less space.
They don't currently use more than 12 channels per socket even though they could because that's enough to not be a constraint for most common workloads, more channels increase costs, and people with workloads that need more can get systems with more sockets. Apple only uses more because they're using the same memory for the GPU and that is often constrained by memory bandwidth.
> Existing servers typically have 12 channels per socket, but they also have two DIMMs per channel, so you could double the number of channels per socket without taking up any more space for slots. You could also use CAMM which takes up less space.
Usually this comes at a pretty sizable hit to MHz available. For example STH notes that their Zen5 ASRock Rack EPYC4000D4U goes from DDR5-5600 down to DDR5-3600 with the second slot populated, a 35% drop in throughput.
https://www.servethehome.com/amd-epyc-4005-grado-is-great-an...
It comes with a drop in performance because there are then two sticks on the same channel. Having the same number of slots and twice as many channels would be a way around that.
(It's also because of servers being ultra-cautious again. The desktops say the same thing in the manual but then don't enforce it in the BIOS and people run two sticks per channel at the full speed all over the place.)
Existing servers typically have 12 channels per socket, but they also have two DIMMs per channel, so you could double the number of channels per socket without taking up any more space for slots. You could also use CAMM which takes up less space.
They don't currently use more than 12 channels per socket even though they could because that's enough to not be a constraint for most common workloads, more channels increase costs, and people with workloads that need more can get systems with more sockets. Apple only uses more because they're using the same memory for the GPU and that is often constrained by memory bandwidth.