Because book quality has nothing to do with release date.

Books aren’t fruit.

And because ”classics” means a huge, multi-century or millennia long filtering process, where it has been found good, useful, and insightful for generations upon generations of people.

Also, because all the best of your modern books will soon enough be ”classics” too, and you’ll be as dead as the people who wrote the older classics. So what makes you think you or your contemporary authors have some unique insight just because you happen to be living at the moment?

Only a belief in an arrow of progress where non-technical things (morals, books, ideas) get monotonically better (or at least, where the dominant vector over time points to better) would justify not reading the classics. But then again only someone ignorant enough to not have read the classics would believe such an idea. So reading the classics will cure you of that too.

(Even for technical things it’s not always the case - there were civilizations more advanced than those that followed them, or periods where we went backwards in quality of life, knowledge of science, etc., for centuries - but at least with technical knowledge it’s possible to amass and improve. You don’t amass morals however, and even the higher moral ideas and ideals can be used for the worst atrocities).