Effectively what Turing Pi, Jetson and similar are, but I don't understand why computers, as a class, are always designed the way they are, and there are no ser

Effectively what Turing Pi, Jetson and similar are, but I don't understand why computers, as a class, are always designed the way they are, and there are no serious LTS module&backplane-based options for SOHO. Everything should scale up by offering all resources in physically being able to plug into something else. I feel like if you just happened to have 15K iPhones laying around, you should be able to readily pool and abstract all their resources for an effective supercomputer cluster... with a bunch of Apple iPhone backplanes, but I don't see them for sale anywhere.