Because it's difficult to measure software engineering competence unless you were involved with a very high visibility project? Most software engineers work on

Because it's difficult to measure software engineering competence unless you were involved with a very high visibility project? Most software engineers work on proprietary code that no one is allowed to examine. So you have no idea if the principal engineer you are about to hire is really good at his craft or not because you cannot examine his prior art, it being proprietary to his previous company.