Because that's the biggest fallacy of all.but as a consumer I've never seen "If you don't like it, change it" pay off for me.
I can submit a bug, but I can't make them fix it. Just a few days ago I received an email telling me that a bug I submitted for Google Chrome has been closed because "it is no longer valid". Even though the bug still exists.
I could write a patch for something, but I can't make them accept it. And writing a patch would require me to be knowledgeable in whatever programming language is being used, and, intimately familiar with the code base so that I know where to find the code that needs to be patched.
I could create my own fork of the program, which I actually tried once a few years ago. But I gave up after awhile because: I'm not an expert C++ programmer, the source code was a mess with absolutely no documentation telling you where to find anything in the 30,000 files scattered across hundred of directories, all of which made maintaining my forked version a huge pain in the butt. The whole thing was way too much work for far too little benefit.
The tl;dr summary: If you are an expert programmer, who is intimately familiar with the code base and who has commit access to the source tree, open source is wonderful. If you aren't, it's meaningless.