how is it a stupid question?
It really is a stupid question... and even if sins were able to use it, the animation would look so screwed up, it doesn't really suit an assassin. I know you could argue that irl assassins can use bows and guns but then again I could tell you that, "this is Ragnarok... not irl". Most classes have at least 2-3 main weapon types, Assassins being Katars, daggers, swords; Rogues, Bow, Daggers, Swords etc...
Even if sins were given bows to use, they have too much at their arsenal, like CMI said, dual daggers can dish about 1k+ dmg whilst with bow you could probably dish out approximately 500 dmg... so it would be pretty idiotic to go for the lower number amount... If you're scared of fighting close range make a bow class for gods sake... Sins have flee for a reason
