Because paying is the problem, and expecting that prospective employers would pay for your expenses for a test that is used to infer basic competences is at best wishful thinking.

We only need to look at the current state of qualification mills/moocs to get a good idea how job seekers would be fleeced by these so called certification systems.

It's far better to get in front of a prospective employer and solve whiteboard problems than just be a number in a clueless HR drone's database.