I think it stems from an intrinsic starting point among many developers that programming is a true meritocracy and that all hiring questions can be settled with a judicious study of what the person is truly capable of (eg. the fabled personal github account filled with side projects). And that anything other than "hacking on the code" is a waste of time.
Of course this isn't how the real world works. In fact almost no one has the time, energy, or inclination to do impressive side projects. Gauging skills on the basis on resumes and very short interviews is quite hard and time-consuming. Hiring decisions are made on the basis of essentially social interactions and personal references.
I do think there is room for an informal credentialing process in principle, but there are longstanding mental blocks that will need to be skirted. There will need to be leadership from the top to make this happen, ie. the best, most-respected programmers will need to take it seriously first.