Amoeba paramecium and spirogyra are diverse. why then are they all classified as protists

Even though amoeba, paramecium, and spirogyra are quite different, they are all often classified in the same group, Protista because this group holds various types of eukaryotic organisms that are heterotrophic and unicellular.

This type of classification is not based on evolutionary and genetic similarities, but only on the properties of being unicellular, so there are many vastly different organisms in this group.