How do you prove to an employer that you've "studied at home?" Why should an employer even start with your resume without some educational credentials that show you've done some critical thinking, are functionally literate, and have shown that you at least have the tenacity to spend 3-5 years in pursuit of something?
And as far as internships, what company would invest the time in training someone without some proof that candidate can learn? This is particularly true for positions in the medical field, law, and engineering, to name a few.
“For that matter, certification tests can be used for purely academic disciplines.”
(From the article.)