Programmers can be unemployed for a wide variety of reasons.

* They’re not very good and can’t pass an interview.

or, they’re good but…

* They can’t pass an interview because their interview skills don’t adequately represent their skill.

* They are not interested in the types of programming work available. For example, Bob wants to be a game programmer on game X and is unwilling to consider game Y or non-game programming jobs.
* They are not going about finding a job the best way, e.g. they apply to jobs on their websites rather than working through contacts, they apply for jobs they’re not qualified for, they don’t have good resumes, etc.
* They are not willing to relocate to where the jobs are.
* They don’t want to work right now.
* They have some attributes that make them hard to employ: don’t play nicely with others, etc.

I imagine a large percentage is the first item, they’re not very good. Just finishing a degree doesn’t make you an employable programmer, any more than finishing a MFA degree makes you an employable writer: it takes skills and talent in addition to that credential.