Because you haven't been playing with me.

Hey, I once allowed a half-dragon hydra as a PC - in the same party as a half-nymph drow paladin of freedom, no less. I'd say the closest to a Drizz't clone there was the human fighter/rogue who dual-wielded bastard swords and murderized via stealth. So long as there's more to the character than "Exactly like Drizz't", I'll let you try it out. Personally, I don't think it'd kill anybody if more DMs were like that. What's the worst that'll happen? It turns out really crappy and we have to suffer through it for a little while before I twist your arm and you make a new character?