CMS's are great but they are software not a static HTML webpage.
There will be updates as the developers refine code, fix bugs, add features and so forth. Sometimes there are even security updates.
To be fair again I only know drupal, which has the reputation of being the most complex of the CMS and it is famous for the fact that Whitehouse.gov is built with it. The joke is the US Government used a free open source program and it cost 18 million to develop. While on the other hand, it is reported that Energy.gov is saving 10 million a year over their old platform.
Drupal likely powers the most robust and complex sites on the internet in the public and private sector. For a simple website it is as Tony's says " hunting Squirrels with a rocket launcher"
That is not to say Drupal or any other CMS is too powerful for the small website because it can and will serve up a few pages of content, provide simple user interfaces and editing tools (for content).