Here's why:

* Organism Complexity: Organisms exist on a spectrum of complexity.

* Unicellular organisms (like bacteria and amoebas) are single-celled and don't have specialized organs. They carry out all life functions within that single cell.

* Multicellular organisms (like plants and animals) are made up of many cells. These cells can specialize and work together to form tissues, organs, and organ systems.

* Organ Definition: An organ is a group of tissues that work together to perform a specific function. For example, the heart is an organ that pumps blood, and the stomach is an organ that digests food.

Therefore:

* Unicellular organisms lack the complexity to form organs.

* Multicellular organisms can have organs, but not all do. For example, some simple multicellular organisms like sponges lack true organs and instead have specialized cells that carry out specific functions.

In short: The presence or absence of organs depends on the level of complexity of the organism.