They don't leave it open at all. The fabled halfling luck causes them to be ignored a lot of the time by bad races, and then when someone does threaten them, they defend themselves and with their luck, they would defend themselves quite ably.
Neither the rules nor the lore say that. It's totally open.
This is an excuse reason given due to halflings not being fully intergrated into the game as an iconic core race but stated to be one.
If halflings are master slingers and duelists, then slings need to be better weapons or shortswords given to halflings for free with upgrades.
If halflings are super lucky, it needs to apologize to 4e and get Second Chance back in the base class rules.