A CMS is not a book. You write a book, you publish it, and there it is, a lovely block of paper. It's done. It can endure for centuries.
By contrast, a CMS is never done. If you don't keep upgrading your CMS, you risk losing your site.
Websites, Like Cars, Need Maintenance
Many people think of websites as a one-time expense. They want to pay a developer, get a nice website "built," and then move on. But that's not how it works.
A website needs to be maintained.