That's true of any large old piece of software, I sometimes read my own code written a few months ago that I've forgotten and need to spend time to understand what it's doing, imagine reading someone elses code written years ago. Companies don't incentive good documentation or comments, and rarely have I seen proper coding standards enforced, so you end up with a lot of spaghetti code, with 600 line methods that do too many things and there may or may not be proper unit testing that covers this code thoroughly

And that's the same reason Linux and open source doesn't, because when developers are empowered to do things that impact them, shit happens