What is the cause of the common impression that European electrical wiring is safer than (North) American?
A few things that immediately strike me.


American plugs have no protection against people or items touching the live pin(s) if the plug is partially withdrawn from the socket. Most European plugs protect against this either through pain insulation or through having the socket at the bottom of the recessed cavity.
Until relatively recently the Americans used combined neutral and Earth connections in some domestic wiring (IIRC subpanels, ranges and clothes driers). If a combined Neutral and Earth conductor breaks you can easily develop dangerous voltages on exposed metalwork. 
While the lower voltage does reduce the risks posed by electric shocks it also means increased currents which can lead to increase fire risks.
Lots more exposed metal inside enclosures, americans don't sleeve their earth wires and their sockets have exposed live screws, it seems the only thing stopping them touching is the neatness with which the wiring is inserted into the box. You can take the lid off a modern European DB and you would still have to go out of your way to touch live parts while the American breaker panels I have seen pictures of have lots of live metal exposed to direct touch.


Of course there are issues on the European side too.


Earthed plugs will often fit in non-earthed sockets or sockets using a different earthing system in such a way that the appliance receives live and Neutral connections but not an Earth connection.
Accessories are often designed in such a way that they are mounted to the plate, so you can't see what is happening to the wiring as you push it into the box.


A lot of other stuff depends on exactly where in Europe you are comparing to, for example here in the UK a socket without a proper earth would be considered virtually unthinkable, but from what I can gather in both mainland Europe and north America they are relatively common.