Here's why:

* Cell walls: Plants have rigid cell walls made of cellulose, which provide structural support and help maintain their shape.

* Chlorophyll: Chlorophyll is a pigment that allows plants to capture sunlight and perform photosynthesis, the process by which they produce their own food.

While other organisms like algae also have chlorophyll and cell walls, they are generally considered to be single-celled or simpler multicellular organisms. The combination of multicellularity, cell walls, and chlorophyll is a defining characteristic of plants.