Why don't Schuko plugs have this orientational bias ?
Number '2' is grooved out, and there is a receiving prong that sticks out to make contact to the groove-exposed metal, similar to how the contact chargers you're probably used to seeing on handheld phones (not cellular, but cell batteries are often similar) -- The important thing to note is that length-wise, 2 comes in contact to ground before either hot or neutral wires do (length-wise), so it's safer on paper than the traditional design.
I imagine there's no polarity in european plugs because the "use a third lead for grounding" solution was chosen right away instead of trying to patch things with forced polarity first (possibly with the prior knowledge of incidents from the U.S. - kind of like how PAL could avoid the issues inherent in NTSC)
edit: looks like my assumptions may be wrong - the Schuko patent predates the grounded NEMA plug patent by 2 years. But who knows how ideas like this spread back then... This is the kind of history I'd love to read a book on.
I remember when polarized plugs started to appear on appliances in the U.S. (probably in the 1970s) and by then the third lead was many years old.