The trailer tail aerodynamic devices are supposed to save 5% on fuel. They remain rare and 99% of those are left undeployed. The trucking companies don't seem to care much about easy cost savings.
Especially for local deliveries- the truck is often half-empty.
The fuel tank of a semi truck is not an important proportion of the weight of the whole vehicle, so it does matter, but not so much.
Still even at interstate speeds a 5% savings pays for itself pretty quickly.
They may not be worth it for short haul trucks, but long haul trucks I bet the savings adds up quick.