Because they didn't do that much and index scores < 30 are very crowded
Ramanujan, Srinivasa, 1920, India, Index Score : 7
Von Neumann, John 1943, Hungary, Index Score : 19
"I close it with the observation of the eminent Polish mathematician, Mark Kac, discussing the Indian mathematician Ramanujan. An ordinary genius is a fellow that you and I would be just as good as, if we were only many times better. There is no mystery as to how his mind works.
Once we understand what he has done, we feel certain that we, too, could have done it. It is different with the magicians. They are, to use mathematical jargon, in the orthogonal complement of where we are and the working of their minds is for all intents and purposes incomprehensible. Even after we understand what they have done, the process by which they have done it is completely dark."