Why are trucks non-level from the factory?

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So it's not bro-leaning when you have weight in the bed.

 

All vehicles are higher in the rear than in the front to compensate for cargo weight.

Trucks more than passenger cars because they're used for towing and hauling.

 

It's both aerodynamics and squat reasons. Lower front end means less surface area is used as a "cutting edge" against the wind. Plus the lower the vehicle, the better the aerodynamics which in turn means better fuel efficiency. Similarly, when you load a vehicle up a lot and the rear squats, the front becomes higher which totally kills fuel economy. And not only because of the weight, but also because more of the underside of the truck is exposed to the wind, creating more drag. Personally, I think it's completely silly to level a truck for these two reasons, but everyone has their own priorities.