I strongly believe that they are a part of the content, just not in the traditional sense. Many times I find more substance in the comments than in the main content. Sure it's great to have more control over which commenting system to choose, but would adding a native commenting diminish that control? Certainly not right? (If you check out ghost's forums, you will find many people asking for a native commenting system)
Yes none of those static site generators support commenting, making them not very suitable for blogging(they dont even support searches). That is why I am not a proponent of static generators as a blogging platform.
If you look at the various options being suggested in the comments here you'll see many of them don't come with comments out of the box. Jekyll, Gatsby, Hugo, 11ty all require you to use a third party service or roll your own.