It is the pain of complying with the license. You have to ensure you have everything in place just in case someone asks for the source code. It isn't hard, but it is effort that nobody wants to do yet you have to. It isn't good enough to say "we use version 1.2.3 unmodified download that from the internet", you actually have to have a copy of version 1.2.3 ready to send out (apparently in case the internet deletes all versions of source code 1.2.3).