I agree 100% with what @gibby says, but I want to add some color to it.
- No one, or hardly anyone, at Harvard or any equivalent research university -- Harvard isn't unique at all in this respect -- gets hired or gets tenure because he or she is a great teacher. They get hired because they are great, influential scholars. Some great, influential scholars also happen to be great teachers, too. It's not by accident, exactly -- some of the things that make scholars influential include clarity, charisma, and communication skills, and those qualities help teaching, too. But there are plenty of great scholars who are not so great teaching introductory lecture courses, and that's where Harvard (like most of its rivals) gets its negative reputation. When you get into more advanced classes, students have more background and more focus, so they can appreciate what makes a great scholar great. That's where you really get the benefit of Harvard's faculty.
- What advances the career of a Harvard professor is producing research and scholarship, and mentoring fabulous PhD students, not explaining to undergraduates for the fourth time what's going to be tested on the midterm or listening to them whine about grades. The graduate students they are advising (a) are effectively their more-than-full-time employees for a number of years, (b) work around the clock to understand what interests the professor, (c) help the professor produce scholarship and enhance his or her reputation, and (d) are in a lifelong relationship with the professor. Also, when the President, or a Cabinet secretary, or the New York Times calls, the professor is going to take the call. Undergraduates can't really compete with that, and it isn't worth trying. What undergraduates can do is fit themselves in to a particular professor's ecosystem, hang around, and learn stuff if they put in the work. That's a skill you have to cultivate if you want to have a relationship with faculty as an undergraduate at a research university.
- Something that is somewhat unique to Harvard: I don't know of any other similar college where the average student spends so little time and attention on classes relative to extracurricular activities. That's not true of all Harvard students, but year after year Harvard seems to select for students who are likely to behave that way. If Harvard professors seem to care less than they should about undergraduate teaching. it's due in some significant part to Harvard undergraduates seeming to care less than they should about classroom learning.