wing attacks are best handled by counter attacks in the center. If the center is locked / static, then there is a much less likely chance for a successful counter-blow in the center, so the K-side attack is less risky (for the attacker).
Also, often in the pirc/sicilian white 0-0-0, so the K-side pawn storm carries no risk to king safety at all.
In the pirc and sicilian defenses, what allows white to often play for aggressive kingside expansion with the pawns? I understand that when the center is locked (as in the king's indian), it is necessary to play for pawn breaks on the wing. Is it white's strong grip on the center that makes this expansion feasible? I know that in the siciliand defense, a well-timed d5 break for black often cripples the white position.