Because only one person in a million is capable of taking the source code of a commercial software product and modifying it for their own benefit in a useful wa

Because only one person in a million is capable of taking the source code of a commercial software product and modifying it for their own benefit in a useful way. When you start a new job, do you look at the codebase and immediately think "Ok, I get this, all I need to do is..."? It's fantasy and wishful thinking to believe that it's even possible for someone to build Windows, MacOS, Photoshop, an AAA game, Chrome(ium), or even a commercial mobile app without a huge learning curve and likely quite a bit of commercially sensitive support infrastructure - never mind make useful changes to them. Adding some custom features to a car is trivial in comparison.