It's true that one cannot teach a skill without content, while it's easy to teach content without considering any skill but regurgitation. And it's also true that an overemphasis on 'skill' allows the system to avoid all the potential political and personal conflict of specific content. (Critical thinking without considering America's racial legacy, for example, or Literature discussions without bringing up god.)

But the most significant obstacle to content knowledge - and everything else in education, really - comes down to our marriage to an age-based classroom. Most kids 'fail' (the only word we use, which is a problem too) classes because they aren't ready, and there's almost no room at all - other than failure - for them to wait, change the subject, try a different class, or anything else.

Every kid is different, many widely so, but we still insist they master every bit of content and every skill at a very specific age. And if they don't, we call them failures.