I'm going to say, based on my own experience last night, is because compared to a modern Mac or Windows DE there's no intuitive control.
I installed i3-wm and logged in using sddm and thought it froze. I noticed it was saying a package was missing in the bottom so I looked into that and saw I was supposed to install the i3 meta package. When I did that and restarted sddm to log in again it still seemed frozen. Mouse moved, but couldn't find any way to interact with it. Tried meta+e to open a file manager and nothing. Meta+T thinking that would open a terminal, and nothing. I gave in and searched for how to do it and it was meta+enter.
I felt so completely lost on a computer for the first time since I was 4 years old. It really made me want to give it an honest shot though, so I'll see if I can read the manual and use it over the weekend.