Because it is just 1 giant blow pipe that shoots a dozen darts rather than 1 Skink trying to shoot a dart and reload multiple shots quickly. (and becasue the rulebook says so )
You won't find a paragraph that specifically says that the Giant Blowpipes don't take a penalty, but a specific ruling isn't needed. The -1 to hit penalty comes from the special rule "Multiple Shot". The Ancient Steg does not have this rule, so it does not suffer the penalty. As others have said, it does get to shoot multiple darts, but it has its own specific rules, not the common "Multiple Shot" rule.