Except large code bases do the same. They regularly die when their ability to be understood drops too low. Even with well organized code, they are pushed to add features until they aren't understood at the deepest level. Once you hit millions of lines of code, even when you spend decades in that code base, you still forget changes you made even if you have an overarching picture. That's ignoring other people working on it all the time. The understanding gets reduced to contracts, types, and interfaces.

And most importantly, humans are more complicated than code. With enough time and knowledge, I can accurately tell you what any piece of code does on a single expression or statement. Humans regularly do things they don't even know for purposes they don't understand.