FWIW another reason, somewhat similar to the low hanging fruit point, is that because the remaining problems are increasingly specialized, they require more years’ training before you can tackle them. I.e. not just harder to solve once you’ve started, but it takes longer for someone to get to the point where they can even start.
Also, I wonder if the increasing specialization means there are more problems to solve (albeit ever more niche), so people are being spread thinner among them. (Though conversely there are more people in the world, and many more scientists, than a century or two ago.)
FWIW another reason, somewhat similar to the low hanging fruit point, is that because the remaining problems are increasingly specialized, they require more years’ training before you can tackle them. I.e. not just harder to solve once you’ve started, but it takes longer for someone to get to the point where they can even start.
Also, I wonder if the increasing specialization means there are more problems to solve (albeit ever more niche), so people are being spread thinner among them. (Though conversely there are more people in the world, and many more scientists, than a century or two ago.)