Question

why a gas exert pressure on the walls of the container

Answer 1

Gases move in random direction no matter what you do,unless you cool it to 0 K which is impossible
fluids when they keep flowing,, when flowing they hit each and everywhere,within themselves and other places. These collisions are perfectly elastic and the energy is passed on to directly the molecule it collides with.
So these gas molecules when colliding with the molecules of the container, exerts the so called “pressure” in the walls in the container.

Answer 2

Because it has mass and volume