The problem is that what Newton and Einstein had to do to prove themselves was (relatively) easy. Especially in Newton's case (thinking about it some more, I'm not gonna argue the einstein point.) . Things that some of these 'great minds' did, people now have to do in normal classes. Heck, on a weekly basis, I have to do proofs for fluid dynamics that people got to use as doctoral dissertation topics up until about 50 years ago. It takes a whole lot more to stand out now than it did 400 years ago.