How is anything else even possible? I spent eight years doing CPU development and even at the time I wouldn't say that I understood how CPUs worked. No one understands how an entire desktop or server CPU works today. They're too complicated. Back in grad school, I had a fairly good solid state & device physics background, stronger than most EEs. Ten years later, if I now want to understand how a modern transistor works, I expect it would take me 1-2 months to get a good high-level picture. And that's just the digital stuff -- I never had a good picture of how analog circuits work, and understanding that would take years.
In terms of complexity, the CPU is much less complex than the compiler or OS I use. For that matter, it's less complex than the browser I'm using to type this comment. If you don't believe that, look at the number of person years that goes into developing a CPU vs. the amount that's gone into any of those pieces of software. Relatively speaking, CPUs are simple. And they're still so complex that no human can understand them.
There's no way to understand the entire stack below you unless, maybe, you're doing materials science work and don't have anything else below you in the stack. And even then, "understand" is sort of an iffy term to use. When I was up to date in the area, there were a lot of open questions on phenomenon that worked but had no proven "why". While I haven't looked into it, it's a pretty safe bet that the total number of such mysteries has increased.
This isn't to say that people shouldn't develop some understanding of the building blocks they're using. Rather, understanding is a difference in degree, not a difference in kind. There's no such thing as understanding everything you use. There's just, at the margin, spending a bit more time on understanding components vs. spending a bit more time understanding how to use components.