Even if they can, testing that is costly. So en entity providing proxy measuring of actual knowledge - the acquisition of the certificate - is valuable, and rational. There are shortcomings, obviously, in each measure or simplified model of every reality. For example old certificates lose informative value. That's why, for example, language certificated such as TOEFL are only valid for some time. There are other certificates, though, that probably have an expiration date for business purposes (e.g. GMAT is valid for 5 years, but it basically measures general IQ and analytic capabilities, which shouldn't change a lot in five years, IMHO).