Absolutely agreed. Software is like the medical profession : beyond the common cold, you need a specialist to get a reliable diagnostic.
Creating an OS is not making a fart app for a mobile phone. Even making games has become a very complex undertaking, what with the predictive network code, world mapping and multiplayer handling, not to mention graphics and object interaction (with or without physics involved). There is no way a single human being can assimilate the particularities of each domain in what is commonly called a triple-A title.
You want code that a single person can understand ? Go for the 2D platformers in Early Access on Steam. That is likely to not be too complicated to understand because a lot of them are written by a small group of people. But if you get the code to Call of Duty, I'll wager you'll never get through it all.
So don't knock Open Source on such arguments. That simply doesn't hold up to reality.