I'm not an engineer but do have some experience with designing and ordering large flat back trucks who's rear always got downright filthy in rain, wet road conditions.
The reason this occurs on all SUVs and trucks and not cars is simple.
The design.
All sedans and coupes are aerodynamic meaning the water, and more importantly the wind blows off the back and away from them.
Large trucks, buses, and SUVs because of their large flat back does not allow water and dirt to flow away from the vehicle. Instead the rear air vortex created while driving keeps the water and the dirt towards the vehicle and pushes it into the vehicle, thus creating the more dirtier rear on large trucks, buses, and all SUVs.
What a top mounted rear spoiler might do is keep even more of this dirty mixture at the rear of the vehicle from blowing up and away, but to be honest I don't see any excessive dirt other than the norm on the rear of my Explorer.