And that's exactly the problem. We must require source code with all publications. The purpose of publications is to expand knowledge. The whole reason we have references is so that others may look at our work and build upon it. Withholding source is completely contrary to the hole purpose of publication.
Except that most research groups don't see publications that they. Publications are one of two things: a way to graduate or a way to obtain tenure. That's a very different set of goals with a very different values and activities motivated by it. Reproducibility is not among them.