Grading the homework is not the same thing as providing feedback. A big red X tells you that something is wrong, but not what you can do to fix the problem or even why that thing is wrong.
I certainly don't want to be misquoted as claiming that mathematics must be learned in a vacuum. Just that access to a large collection of questions and answers is not helpful. Your own argument goes through just as well to show that access to the answer isn't helpful either: you know you got the wrong answer but you don't know why.
It is cryptic at times bit ultimately developing rigor and intuition is something that in my experience will always involve a certain amount of struggle and confusion that no professor can or should alleviate