Question

why do you think pinna is immovable in human beings unlike in most other mammals

Answer 1

Pinna is immovable in human beings whereas in case of most other mammals like that of cat, dog, horse, it is movable.

Usually the other animals move their ear towards the direction of the sound.

A group of muscles named auriculares are responsible for this.

But in case of humans, ear movement is a vestigial feature or a trait which was useful in ancient time but now they have become function less with time.