Animals donot have plastids because they don't need them.
If animals had plastids, then they have no need to feed on plants or fruits for food.
If this happens, plants cannot pollinate, seeds will not be dispersed and plants will not be controlled from excessive growth.
So, animals are created as heterotrops, so that they can control food chain and hence control growth of plants and othet animals.
So, plants lack plastids, so that they are forced to depend on plants for food, and in turn pollinate flowers and disperse seeds.
Also, if animals had plastids, we would all look green. It wouldn't be good to see everyone green... Right?