Because you risk missing something interesting or important because of your own arrogance, which is tragic, and most likely reinforcing that arrogance, which is even more tragic.

As the parent mentions, engaging with primary sources from a different historical or cultural context with some patience and forbearance is particularly important for understanding philosophical works, but it’s not that different for say mathematics or physics. Notation and terminology changes. With your attitude you’d probably assume archimedes, newton and Fibonacci were all stupid.

Why would anyone care what, say, Charles Babbage was trying to do, when he didn't actually do it in any sort of useful way? Surely this is largely how knowledge is produced—over time, with lots of false starts, and language and technique that is not yet adequate to the problem at hand.