Words change. Languages change.
English shifted dramatically over the years.
Here is an interesting lesson on the development of you.
Often you will hear people say things like, “I can’t read the King James Version of the Bible. I get tired of the formal ‘thee’s’ and ‘thou’s.'”
Funny thing is, those words are not formal. They are informal. In Old English, you would have used those words with close friends or relatives.