Here's why:

* Sponges are the simplest multicellular animals. They lack true tissues and organs.

* Their cells are loosely associated and perform various functions. They have specialized cells like choanocytes (for feeding) and amoebocytes (for digestion and structure) but these cells aren't organized into the complex, coordinated structures found in tissues.

All other animal phyla (e.g., cnidarians, flatworms, annelids, arthropods, vertebrates) have tissues that are organized into organs and organ systems.