A lot of that is just that English along with much of western vernacular wasn't given standardized spelling until fairly recently, as most of the important writing was done in Latin.

If you get past the weird spelling it's still fairly understandable.

Exception being maybe stuff like Shakespeare, but a huge part of what makes that inaccessible is that his writing is full of references to current events, double entendres, and various 17th century memes. It's a bit like showing South Park's world of warcraft episode to someone from the 2400s.