For the same reason that "yet" doesn't mean "already" in this context. "Has it happened yet?" is asking for confirmation that "it" has yet to happen, whereas "has it already happened?" is asking for confirmation that "it" has (already) happened. While there is uncertainty, yet implies anticipation, whereas already implies that the event is (probably) in the past.
Ar thárla sé fós? - "Has it happened yet?"/"Did it happen yet?"
Ar thárla sé cheana? - "Has it already happened?"/"Did it already happen?"