It's not that they're not paid to care, it's that they're not paid to teach. At least at research universities, tenured/tenure-track faculty's job is to do produce research and get grants. Teaching is a thing that's piled on top of it that doesn't help your career and takes a whole lot of time away from your primary job function.
A lot of professors do actually care about teaching and genuinely want to help students, but the system as designed strongly disincentivizes (or actively punishes) doing more than the bare minimum. I know a professor who was recently denied tenure at a research university that likes to describe itself as very "undergraduate teaching focused" -- he had decent but not outstanding research output, but had gotten several university-wide teaching awards and was broadly considered by students one of the best lecturers in the department. Some of the comments he had received suggest that this actively hurt him for tenure, because they felt he was too focused on teaching over research.
A lot of professors do actually care about teaching and genuinely want to help students, but the system as designed strongly disincentivizes (or actively punishes) doing more than the bare minimum. I know a professor who was recently denied tenure at a research university that likes to describe itself as very "undergraduate teaching focused" -- he had decent but not outstanding research output, but had gotten several university-wide teaching awards and was broadly considered by students one of the best lecturers in the department. Some of the comments he had received suggest that this actively hurt him for tenure, because they felt he was too focused on teaching over research.