But receiving lectures and coursework equivalent / comparable to those received by regular students, such that would realistically prepare you to passing the same kind of exam (given adequate study effort from you), would be actually useful. Useful even if you don't take the exam and don't receive credits / papers. Study is not for costly signaling alone.
What certifying body would administer the exam? A university that would miss out on $30,000 to $200,000+ on a student attending classes? A government institution influenced by politics that would likely end up creating inadequate testing leading to irrelevant examinations disregarded by most employers or anyone of substance?
The only reason why exams work for trades is because it is very well defined what a plumber needs to know. Even for software engineers, certificates are useless for most because what engineers need to know is rather abstract or highly dependent from job to job
If you just want to take a class, there are plenty of MOOCs that give the lectures, exercises, and tests out for free.
Another reason is that different universities may emphasize different things as part of the curriculum. Lets say a philosophy degree at harvard emphasizes Greek philosophers, but a philosophy degree at UT emphasizes post modern philosophers. Taking a class at one doesn't transfer to another. Mixing classes at different universities simply doesn't work because you weren't educated at the university so why should you get a degree from that university?
The way I see it is that if you just want to get educated the resources are out there, but if you want degree, you gotta go to school.