Most professors decided on their occupations as undergraduates. They get excited about learning, maybe get to know some of their professors well and think that teaching college looks like a good job--the life of the mind, working with motivated students, maybe writing books. So far, so good.
Then graduate school comes along, and the world shrinks. The amount of time spent in class declines. The amount of time spent around people not like you declines. In fact, the amount of time spent around other humans of any sort declines. The initiate is being socialized, and she or he is also competing. Our aspiring professor is likely going to a very strong graduate school. Getting scholarships or other aid is often competitive, and so is getting strong recommendations.
Graduate students have three ways to distinguish themselves from the pack: 1) Stand out in seminars by offering brilliant insights in your oral and written work; 2) Ace your field exams in your major and minor field; 3) Write a brilliant dissertation and get an article or two published. The third point is by far the most important. All of this requires a great deal of time on one's own, reading and researching and writing.
One may grade some papers, lead some discussion sessons, maybe teach some classes. But theses tasks are a means to an end, a way to fund one's "real work": the original scholarship that will enable you to both graduate and to have a decent chance of getting a job after you graduate.
The graduate student will also notice that her or his mentors, the professors at these prestigious universities, do not teach very often and may not teach very well. But the great majority of them care deeply about their research, and they derive a great deal of status and pleasure from their publications. One may even tell you, as the head of graduate studies once told us, not to let teaching get in the way of scholarship. Indeed, few graduate students (outside, I suppose, of schools of education) take classes on teaching. There just isn't time for that sort of thing.