This is why they pay fees and make donations, which go toward paying instructors and staff. Teaching develops relationships. Some of the most profound and long-lasting friendships are between instructors and students. Teaching produces future researchers. The beginners we teach today may be award-winning academics tomorrow.
Teaching produces educated people.
College instructors, after all, have as much to complain about in the state of the world as anyone else. Usually, people who went to college. Teaching energizes research. Discussions with students who ask naive questions and attempts to help people understand fundamental concepts can result in new visions and directions for research. Teaching is good experience.