On the contrary, there is a lot that is not rewritten at all.
For starters: The Unix that I (and quite a lot of other people) use today shares entire manual pages with the Unix of more than 30 years ago.
An example: The manual page for the ul command that was written by (then) Mark Horton is pretty much unchanged today in FreeBSD and NetBSD. For 34 years, it hasn't actually described the command correctly or fully.