It's no secret that the -update option hasn't done much in quite a while. This is not because the Nikto project is dead or idle... it's simply because the update/release process requires manual work from humans (there is an open ticket on replacing the update system entirely).
In the meantime: run Nikto directly from the git repo.
This is your best bet for keeping completely up-to-date, benefitting from the latest checks and enhancements, and keeping your installation running smoothly.