Single click to open is great if what you want is to open the folder or file, but what do you use then when you want to select the folder or file without opening it? I've resorted to "vaguely dragging a square around the icon" (to select a group of icons containing just that single icon) on systems with single-click-to-open.

Historically, what you had was single click to select, then something else (usually the Enter key or similar) to open the selected item; this is consistent with for instance drop-down lists (where Enter activates the default button of the dialog box containing the drop-down). Double-click was just a shortcut to "select and then do the default action".

A sibling comment compared to smartphones, which use long-press to select; in my experience, both double-click and long-press can be hard for some people (for instance, I've seen people release the long-press a millisecond too early, causing it to do the single-press action instead of the long-click action).