Those only require you to understand them because you’re working directly on top of them. If you were writing a filesystem driver you would absolutely need to know those details. If you’re writing a database backend, you probably need to know a lot about the filesystem. If you’re writing an ORM, you need to know a lot about databases.

Some of these abstractions are leakier than others. Web development coordinates a lot of different technologies so often times you need to know about a wide variety of topics, and sometimes a layer below those. Part of it is that there’s a lot less specialization in our profession than in others, so we need lots of generalists.