Sure, and some companies strongly discourage looking over other open source code for exactly that reason. But companies don't release their source code mostly

Sure, and some companies strongly discourage looking over other open source code for exactly that reason. But companies don't release their source code mostly for a different reason: they honestly believe that this is where their value is, or they believe support costs would outweight any benefits. For some, it's true, but for most, it's utterly false (or there is a tiny little bit of code somewhere that is valuable, with all the rest being boring code that nobody else could put to use).