Can you give one single example of a mental/intellectual task that humans have generally gotten better at in the last 200 years?
Reading? Arithmetic? Prose composition? Everything in a high school curriculum? Not to belabor the point, but 200 years ago was the early 1800s.
We've built upon the collective knowledgebase, true. But it is as hard for a person today to do arithmetic and logical reasoning as it was in the 1800s.
then why do kids still struggle at algebra and other ancient concepts
More kids learn those than before. But that is better explained by everyone spending thousands of hours learning math today, kids spending that much time on math 200 years ago likely learned algebra as well.
Ancient kids struggled less with algebra by not being expected to learn it at all...