Even so, it is used in the majority of European countries and usually it is more important than the Birthday! Quite surprising as it could be an optional feature.
Anyway, a short search has brought forth a plethora of FB apps providing just this functionality; not sure why it should be implemented in "core" FB.
(And the obligatory disclaimer: even all European FB users wouldn't even be a simple majority of all FB users; building in such a feature for a fraction of users would inevitably lead to other factions crying for their own specialized metadata, leading to fragmented FB experience)