Why don’t all animals have an excretory system?

There is a wide variety of animals. Some animals have a fairly simple structure of their body. Many animals have no nervous systems, no respiratory or excretory systems, and some animals have very primitive systems. All due to the fact that they do not need them because of the simplicity of the body. It is possible to excrete unnecessary substances, water residues, toxins and organic waste through the entire body surface with the help of cells. But most animal organisms have an excretory system in their bodies.