I frequently see line-of-business software that's likely self-explanatory for a 20-something developer, but that's nearly incomprehensible for the target audience. In terms of basic computer literacy, most developers are far above average.
For example, a friend of mine had to teach his elderly coworker at a machine shop about copy/paste. Not the keyboard shortcuts, not how to do it, literally the concept of the system clipboard. He had learned computing on DOS, and had learned everything since by rote memorization.
That's a rather extreme example, admittedly, but it does illustrate just how unpredictable the skill level of end users is. Even a minor UI redesign can throw off someone like that for weeks, and I don't think most devs fully appreciate that.