While I can't speak for anyone else who writes tutorials, I don't include keyboard shortcuts for three main reasons:
1. Software on different OSes can sometimes use different shortcuts for the same functions. This means that the tutorial writer would have to include text such as "(Cmd+F on Mac, Ctrl+F on Windows, Alt+F on Linux)" wherever a function was mentioned. This makes the text more difficult to read and it stops the flow. (And sometimes different Linux distros use different shortcuts for system-level functions which adds another level of complexity again.)
2. In my experience, most new users - the people who tutorials are mostly for - don't use keyboard shortcuts. It's easier to remember "Edit - Find - Replace All" than Cmd+Shift+R (or was that Cmd+R, or Cntl+Alt+R, etc.).
3. In versatile software, such as Scribus, people can change keyboard shortcut bindings. So users inheriting an installation from another user may think that the software isn't working when they use a shortcut and it doesn't work as mentioned in a tutorial. This can frustrate people and put them off from using the software properly. (Quite a lot of people change the shortcuts to reflect how InDesign or Quark Express work, for instance.)
My general view is that people who want to use keyboard shortcuts will find out what they are and people who don't want to use them will ignore them.
Most documentation will include a full list of default shortcuts for those who want them anyway.
Having said all that though, and in this case specifically, if there's something missing from - or wrong with - the Scribus documentation then please create a ticket on Mantis and someone can look into it. It's always better to raise the problem rather than expect someone else to. And it gets you more involved with the community too, which can't be a bad thing.