Here's why:

* Unicellular organisms consist of a single cell that performs all the functions necessary for life. Examples include bacteria, amoeba, and yeast.

* Multicellular organisms are made up of many different cells that work together to form tissues, organs, and organ systems. Plants have specialized cells like xylem and phloem for transporting water and nutrients, chloroplasts for photosynthesis, and roots, stems, and leaves for different functions.

While some algae, which are closely related to plants, can be unicellular, the majority of plants are multicellular and complex organisms.