The trailer is meant to be paired with a compatibly designed tractor unit leading to greater fuel efficiency of the tractor-trailer combination.
As such, the "teardrop" phrase refers to the entire combination, not just the trailer.
The curved shape of the roof decreases aerodynamic drag by smoothing airflow over the top, thus improving fuel efficiency.
[1] The design echos streamlined shapes used in submarines, automobiles and camping trailers.
Improving tractor aerodynamics and using low rolling resistance tires were adopted at much higher rates.