- the state of the art is both extremely diverse and rapidly moving, so any credential system would run the risk of being too specific and easily out of date. Very few are respected and most existing ones are tied to particular products. Possibly the only set of those that is respected is the Cisco ones.
- most of our work is (a) tightly intermingled with the work of others and (b) copyright belongs to our employers, so we can't build portfolios like artists. Many people have contracts that purport to own all code produced, even in your spare time.
It would be great if we could take all the basic tests once, upfront, under exam conditions, and then re-use that for years. I can't see how it can be coordinated. Maybe what we need is not a union but a professional body. In the UK we sort of have one (BCS, and more absurdly, the Worshipful Guild Of Informational Technologists) but their impact and benefit is also pretty tiny.