I really don't think it's a matter of "haters". It might be more logical and consistent if you have no other frames of reference, but most Plasma users come over from other OSs who all use double click (Windows, Mac, even Gnome). If a new user blindly tries KDE and keeps accidentally opening everything while trying to select it's just an immediate and big annoyance. It's not even clear that it isn't a bug because there is no clear explanation of how to select and how to open.
Edit: we are of course all used to single clicking on touch screens, but there it is contrasted with the long press to see options and some "select mode" for file management. There is no system that works exactly like Plasma single-click, which makes it disorienting.