You ask: "Why does a professor at an IIT think that cracking the JEE is easy?" I don't know which particular professor at a particular IIT thinks this way, so, I can't answer for him. But, if you want to ask about professors at IIT in general, I can answer that.
Professors at IIT think that cracking JEE is easy because it IS easy if you have the right aptitude and sincerity. You don't need to slog for hours, if you easily get the concepts. The example of Virender Sehwag comes to my mind whenever I think about effortless "success." I don't think he had to work very hard to get into the Indian team. It all came naturally to him. He did not have to work very hard to crack into the Indian team or to maintain his position in the team. He is a "see the ball, hit the ball" kind of a guy and it is so exciting to watch. Similarly, one who gets science, it is just as effortless. Obviously, some sincerity in preparation is required, but, it is not enough if you do not have the aptitude. If one doesn't have the aptitude, he has to work very hard to get in, work harder to stay in and work even harder to get a job and maintain a job. So, if you think that getting in is very hard, I would not recommend this path to you.