Because in 1450, the english language was a lot different compared to modern day english making it difficult for modern students to understand. Also, Shakespeare uses a lot of figurative language.
One, Because it is written in old English. Secondly, Shakespeare invented many words which he used in his plays and the exact definitions are not known. The majority of the terms used are from hundreds upon hundreds of years ago and the 21st CE uses modern English. I don't think Shakespeare would know what "Lit" and "Fleek" would mean if he were to come back.
