to qualify, unions may have solved this problem in the US. in europe, which has a stronger apprentice culture, unions don't have any role here.
(i am not saying this to criticize the comment but just to point out one of the many differences between unions in the US and europe)
the problem with apprentice programs is that they lag behind the changes in the industry. when i was in school there were none for software developers for example. there are now, but the second problem is that apprentice programs are considered of lower status compared to studying at a university. think blue-collar programmers vs white-collar software developers. the pay is also different.
i don't know anything about union accreditation programs, but i can imagine that they would be targeted at the available jobs, and with unions in the US having more influence at who gets hired, they probably can make sure that an accreditation actually leads to a job.
apprentice programs traditionally also promised that you'd get hired at the company where you learned, but this is no longer certain. together with the status and pay differences it is no surprise then that the number of people starting apprentice programs is declining.
on the other side university studies are not targeted at the industry. which only adds to the perception of candidates not matching employers needs. as an employer it is difficult to tell whether a candidate with a diploma is actually capable of doing the job.