Who said anything about loss of quality? I'm not even necessarily advocating abolishing licensing tests!
Because tests are generally easier to game than courses and courses are generally easier to game than full curricula.
I actually had a student do that several years back. My university requires foreign language proficiency for graduation, which can be fulfilled by taking/passing a class at the second year level, or taking a standardized test on a computer. He knew not a word of Spanish, but simply took the test a few dozen times, until he was able to get a passing score by chance.
The next semester they changed the rules, limiting students to only three attempts at each test or so.
He couldn't have pulled that off in a Spanish class; a competent teacher would have known he was faking it. He certainly couldn't have pulled that off in multiple classes; while one teacher may not be competent, they're not all incompetent. And if they were all incompetent, the accreditation board would have noticed and complained.
If licensing was based on a system of tests that were effectively monitored for cheating, how could there be a loss of quality.
You've answered your own question. It's harder to monitor tests for cheating.
And
that's what the accreditation system exists in the first place. Programs exist to contextualize and to validate individual courses and teachers. Universities exist to validate programs. Accreditation boards exist to validate universities -- and the good accreditation boards, ultimately, are validated by the Department of Education.
You're asking instead to skip all that pesky quality control stuff and do an unvalidated course of study, to be confirmed by an inspection at the end. That doesn't work in industry -- we're well aware of that, which is why high quality products are validated early and often, and the
process by which the products are produced are validated independently. If you wanted to make a drug, but refused to allow the FDA to see the process by which you made it and only allowed them to test samples at the end of the manufacturing process, they'd throw you in jail.
But, on the other hand, apparently if you just want to
prescribe drugs, there's no need to validate the quality?