Because people try to read it like modern Chinese. Modern Chinese is written to be read quickly: read it as quickly as you can and go on to the next article.
Classical is written to be read slowly, out loud, sung, chanted (and not recited). Words were chosen with great care. You have to slow down and savor each word to understand how it is being used, how it resonates (典故), how it fits into the article. Then read the article again. And again.
If you can read modern Chinese but can’t read Classical, you are reading too fast. Slow down. Think about the words. If you can read modern Chinese, you can figure them out, and if you can’t figure them out, why, look at the notes or check a dictionary, or pretend you understand, nod sagely, and keep going.
Classical Chinese is not difficult to learn at all, but it does take some getting used to. It takes some practice. But once you get used to it, you have thousands of years of literature on all topics imaginable. You can spend your life on it and barely scratch the surface. It’s definitely worth the effort.
(disclaimer: I may not be totally disinterested in this matter.)