Bescause they are very lossely packed .. Because gas can spread throughout the container in which it is placed. If the container is large, it can spread widely and if the container is small, it cannot. So gases do not have fixed shape or volume.
The very weak intermolecular forces can not hold the molecules within a certain volume, so they move in all the directions in a closed container or in atmosphere.
they dont have fix volume and shape because there molecules are free and and can move freely.
they do noy posses and property of stiffness .
different gases have different volume because thre mass no. are different