Windows can be set to only require a single click... I think that's actually since 95, but I'm not sure; it's definitely at least since XP. Did the default get fixed at any time, or does Windows 10 still default to require double click?
The default on Windows 10, Gnome, Unity, KDE and Xfce is double-click to open file. Otherwise, how would you select an file without opening it?
KDE always had single click file opening/launching/etc - even from KDE1 days. In old KDEs -IIRC- you select stuff by hovering them for a second or so, in new KDEs when you hover an icon there is a "+" sign over it and if you click it, it adds the file to the selection (and becomes a "-" sign). You can also simply Ctrl+Click or Shift+Click on things to select them, only plain click opens/launches a file.