Earlier quoted context omitted.
You've said shitty and complex code...how about voluminous code, or old code? The main product that I work on has about 2 million lines of C, C++, and Java code in the core of the system, then about 20 plugins ranging in size from a few thousand lines to a few hundred thousand lines. We basically hire for sections of the codebase. It's pretty common for developers to discover the existence of an area of code that the…
Why have everyone know all of 2M lines of code? Programming is about abstraction, knowing what a subsystem does, what it's responsible for should be adequate most of the time until you need to fix something inside it. We aren't doctors, we just need the skills to figure things out and fix them quickly.
My point was that someone can ramp up quickly on a portion of the system and have a vague idea of other parts of it, but "mastering" it covers too much code to do it in a reasonable time. Each plugin introduces special cases for the data flow, and the plumbing for that ends up running through the whole system.