I wonder if 20, 30 years ago there was any incentive for the math guy to try things harder
You should try to read some book of cryptography written 30 years ago, they are more close to something that US
Example:
A Course in Number Theory and cryptography by Neal Koblitz
That have more complex and cool Math exercises to solve, that our rudimentary calculations.
But yes they don't have at that moment our incentives, but they still did a really good job.