[m12k]

I think part of the reason is that when you choose to get a degree, you don't know any better, and it probably seems as good a way to get into the field as any (in lots of other fields, it's the only way in - e.g. bio-tech, so it's an easy assumption to make). By the time you get far enough in your career to get a certification, you're also far enough along to know that it's worthless, so it's more of a red flag to others if you choose to do so anyway.