Well, for sure it's the software that's the problem. But since the computer is essentially worthless without the software, the two are an inseparable assembly in practice.
My theory is that there is only a fairly constant fraction of people who are cut out by nature to design things. Attention to detail, ability to foresee unplanned consequences, ability to focus on the actual requirements for a product, patience with the inevitable test-fail-fix-retest cycle, and many other personal characteristics are typical of people who are good at designing things. Whether the "things" be cross-country power transmission lines, automobile transmissions, jet airliners, or computer software, the basic principles of how to design things, and the requirements of their designers are similar.
Now if we go back to 1950, there were all kinds of things that had to be designed - printing presses, locomotives, highway bridges, bicycle tires, etc. My theory is that in those days, pretty much everyone who was capable to design things, was employed, designing things.
Once the 1970s and 1980s hit, a whole new category of things that needed to be designed opened up: computer software. Of course software comes in several flavors - consumer-grade, professional use, and embedded software for critical systems, with high reliability required. BUT THE NUMBER OF GOOD DESIGNERS IN THE POPULATION DID NOT MAGICALLY INCREASE. So today, with far more "things" requiring designing (the locomotives, highway bridges, airliners, medical devices, etc., are still out there needing to be designed), a large number of people who just AREN'T VERY GOOD AT DESIGNING THINGS are now designing things.
Well, a design fault in an airliner, or a highway bridge, or a medical device, causes people to die. The companies that make those things (as well as mission- and safety-critical software) work hard to make sure their designers come from the people who are really good at designing things. And what that means is that the consumer grade software companies have to select their designers from the knuckleheads that are left over.
Very many of the exasperating mis-designed characteristics of modern consumer grade software bear, to the experienced eye, all the hallmarks of being designed by people who JUST AREN'T VERY GOOD AT THEIR JOBS. Constant changes in UI; the fact that every single update introduces new bugs into functions that worked fine before; constant churn in features; constantly increasing complexity in the keystrokes and actions to accomplish simple tasks; these all look very much like product designs coming from people who have little perspective on how to design things, how to guard against unforeseen problems, and how to resist the "oooh shiny new" executive plan-du-jour syndrome.