It's the same reason why Bards deal less damage than other classes. Bows have historically been one of the most terrifying weapons with unrivaled stopping power for thousands of years. Yet in this game they deal less damage than any other melee weapon by a wide margin. And i'm talking on a per hit level, not combos. Doesn't really bother me than a monk that hits 3-4 times per attack in a 4 attack chain deals more damage than a single hit, but the autoattacks...
And it's looking like machinist might end up in the same boat...