The issue is not that nobody can hold these systems in their head, they can - they're just not paid to.
Your average developer will be working in a new job, on a brand new tech stack, in 2 years. Is two years enough time to understand the intricacies of doing serious work with that set of tools?
The additional bit is that operating system kernels and hardware have gotten really, really, really good - so very much of the programming paradigm is to spend as little effort as possible to write the stupid thing, because most work is boring and stupid and doesn't get you a competitive edge, eventually the compiler, the operating system, or better hardware will fix it for you. However, if you spend a lot of time working through something and doing it right, the timeliness gets you put on performance review.