Because it requires no ongoing effort or supervision to be effective, and it can be discontinued only after some (rather small) effort.
It doesn't require an ongoing effort to be effective.
It requires an ongoing effort to be effective.
The children have taken over the classroom and are running around causing chaos
The cat is roaming through a kitchen.
Nobody is lying down.
The man is sleeping with an expression on his face.
Starr said that he has exonerated Clinton.
The woman brushed the horse.