The ability to “share memory faster” is a bigger distinction than you make it out to be. Distributed applications look quite different from merely multithreaded or multiprocess shared-memory applications due to the unreliability of the network, the increased latency, and other things which some refer to as the fallacies of distributed computing. To me, this is usually what people mean when they talk about “horizontal” vs. “vertical” scaling. With modern language-level support for concurrency, it hurts much more to go from a shared memory architecture to a distributed one than to go from a single-thread architecture to a multithreaded one.