Why have protists been "mixed in" with plants, animals, and fungi under the hypothesis that groups eukaryotes into five supergroups?

The Protists remain a very miscellaneous group of organisms that are made up of all eukaryotes that are not fungi, animals or plants.

Multicellular protists such as seaweed do not have highly specialized tissues or organs, which distinguishes them from true plants or other eukaryotes.

Many protists do not have much in common with each other; Some are more closely related to plants and animals than other protists.

Explanation:

Protists are a group of all eukaryotes that are not fungi, animals or plants. As per result, this one is a very diverse group of organisms. The eukaryotes that make up this kingdom, the Protist Kingdom, do not have much in common with each other, in addition to a relatively simple organization. Protists can look very different from each other. Some are small and unicellular, like the amoeba in the Figure below, and some are large and multicellular, such as seaweed, in the Figure below. Though, multicellular protists such as seaweed do not have highly specialized tissues or organs, which distinguishes them from true plants or other eukaryotes. In addition, unicellular protists may be little more than a single-celled nucleated organism.

Many protists do not have much in common with each other; some are more closely related to plants and animals than other protists. Kingdom Protista has been compared to a "trash can" kingdom, in which all eukaryotes that are not plants, animals or fungi have been "thrown".