Only ThreadRipper Pro mobos support that much as far as I know. ThreadRipper Pro CPUs are AMD EPYC server grade CPUs that are customized/rebranded for the workstation market segment. The regular “gaming CPUs” and motherboards support only 64-128 GB of RAM. Some have 4 DIMM slots, but the CPU still has only 2 channels, so with 4 DIMMs you won’t get any memory access throughput increases, just more space for caching stuff to avoid disk IOs. EPYC/TR Pro have 8 memory channels…
Every TR has at least 4 channels
Because they're intended for another market, where people do use that. You don't buy a Mac Pro to use as a server, for example...
Some would like to use it as a iOS / macOS CI server. So server hardware features designed for a data center & rack mount options would be appreciated. Yes you don't need the 2TB of ram in that case, but other parts would be helpful!