Why do French power sockets have a protruding ground?
I'm not 100% sure, but I believe that the reason for the unusual earthing arrangements on french and german plugs is backwards compatibility with existing installations.

Appliances can be categorised into 3 categories.


Class 0: no earth connection, not "single fault safe" (now banned in most first-world countries)
Class 1: "single fault safe" only if an earth connection is present.
Class 2: no earth connection, "single fault safe" 


If your existing installed base of installations has no earth provision and your existing base of appliances is class 0 then it can be argued that it is sensible to let people plug class 1 appliances into their existing unearthed sockets. They aren't any less safe than the old class 0 appliances were.

There is also an argument to be made that having some appliances earthed while there are class 0 appliances in use makes things less safe than having no earthing at all.

Adding a conventional earth pin disallows plugging new appliances into existing installations, and does nothing to prevent old class 0 appliances from being brought into new earthed installations.

The design of the German and French plugs on the other hand allows new appliances to be used in existing installations in a way that is no less safe than before, while keeping old class 0 appliances out of new installations.