Obtaining a degree should be a separate optional examination, likely on-site, like other extern examinations.
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.
I agree that the education system would be better with this kind of arrangement, but it doesn't happen because of economics and american independence.
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
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.