Many of those artists are in the Rock Hall as early influences and not performers, but your point is still a good one.
The concept of "rock" used to be a big tent. For example, I don't really have a problem considering much of the pop music of the 80s (Madonna, Prince, George Michael, even Michael Jackson) "rock". That changed with hip hop and then the stripping of guitars out of most pop music.
Anyway, today it seems like some people only consider rock music to be hard rock and heavy metal. For example, someone in this thread questioned whether the Beatles were rock. Of course they were!