It's not a technical issue, it's a "chicken and egg" market issue. Many desktop applications _would_ run very well on massively parallel hardware, but that's not what people have, so it's not what developers target. And since games are written not to use more CPUs, people don't buy computers with many CPUs. And because MPP hardware is a niche, mainstream developers have no idea how to program for them, much less to think about what problems would run well in parallel.
From a technical perspective, which I think Linus is trying to argue from, many desktop applications could easily take advantage of massive parallelism. Once you start thinking in terms of data parallelism or agent parallelism, almost all problems decompose in ways that parallelize nicely. For example, there are hundreds of AIs and simulation objects in many games, and each could run on a CPU (or process or thread). Video and image processing are "embarrassingly parallel", and now that people edit video at home, they could happily consume all the CPU you have. Sorting, searching, indexing, scrolling in documents, rendering characters to the screen - all very parallel.
Luckily the "graphics processors" are breaking out of the "chicken and egg" trap. The better GPUs are now not really "graphics processors", they are fully general MPP CPUs, and many applications are taking advantage of them. Interestingly this architecture is similar (at a high level) to the MPP supercomputers from decades ago. The Thinking Machines' Connection Machine had a fast front-end computer, controlling an array of thousands of tens of thousands of CPUs that did the heavy lifting, and now it's your CPU controlling an array of CPUs in your "GPU". So millions of PCs are MPP, even though their owners probably don't think of them that way. And this is leading to more and more applications taking advantage of MPP!
So I think that Linux is wrong, in that he's missed that what he's dismissing as GPUs are actually MPP co-processors that are astoundingly powerful and are increasingly being taken advantage of by developers when performance matters.