Here's why:

* Protists are a diverse group of eukaryotic organisms that are not classified as animals, plants, or fungi.

* They are often single-celled, although some can form colonies.

* Examples of protists include amoebas, algae, and protozoans.

It's important to note that while many protists are single-celled, some are multicellular. The defining characteristic of protists is not their cellular structure but rather their lack of specialized tissues and organs.