Because it would be tremendously difficult to grade the students on both things separately.
One of the problems with schools and our society in general is the idea that you can take something complicated and distill it down to a single, meaningful number. Another problem is the notion that you should even try to do this. We're taking this to ridiculous extremes.
I can easily say in a single sentence, "Johnny has mastered 95% of this material but he's apathetic, only completes half of his homework on time, and throws spitballs at his classmates." Done. How hard is that?
Are they trying to save paper and ink costs on report cards?