Why the animal cell does not have a cell wall?

Plant cells have cell walls while animal cells do not have cell walls because they do not need them. 

Plants need a rigid structure to support as well as to defend themselves whereas, animals do not possess any type of cell wall because animals can move, they are not static in one particular place, therefore they need to be flexible.

This rigidity acts as an exoskeleton and allows plants to stand upright without.

If animals would have cell walls in the same way as plants, they wouldn't be capable of movement.