It doesn't "always" cost more. But there's no discounted Saver inventory bucket for premium economy, only "Everyday" awards (ON). So if you're looking at a Saver business class ticket (I/IN) vs an "Everyday" premium economy ticket (ON), the PE award will price higher. When comparing "Everyday" awards, business class (JN/ZN) will usually (but not always) price higher than ON.

Speculation, but UA likely doesn't offer "Saver" PE economy because PE is targeted at business travelers, not those redeeming points for leisure travel.