Why are bacteria, fungi and animals attributed to different kingdoms, although they all feed on ready-made organic matter?

There are many different and unique creatures on our planet, which are very different from each other, and therefore different organisms have been identified in different kingdoms. All living organisms are divided according to the type of nutrition into heterotrophs (they eat ready-made organic substances) and autotrophs (they themselves extract organic matter from inorganic substance). The latter include plants and some types of bacteria, and the former include most of the bacteria, all animals and fungi. But despite some similarities, each kingdom has significant differences.

Representatives of the bacterial kingdom
This is the oldest group of creatures living today. Bacteria can be found all over the world. They settled in the deepest depths of the oceans, in hot sulfur springs, in the mouth of a volcano, in the cold ice of the Arctic, etc. All representatives of this kingdom are unicellular organisms. By its structure, their cell has significant differences from the cells of other organisms. For example, organelles of a bacterial cell include:

cell wall,
flagella,
cytoplasm,
nuclear matter.

The cell wall performs protective and supporting functions, the flagella and villi serve as the organs of movement. The cytoplasm has a very dense texture that contains nutrients. In the center of this cell there is an accumulation of nuclear substance, which contains hereditary information. These organisms are classified as prokaryotes because they do not have a nucleus. In unfavorable conditions, bacteria form a spore in which they can remain for a very long time.

Representatives of the animal kingdom
Creatures that belong to this kingdom can have both unicellular and multicellular structures. But the cell of any represented organism has its own distinctive features from the cells of other kingdoms. It does not have a dense cell wall. There are no vacuoles in it. Glycogen accumulates as a reserve nutrient. And the animal cell has a cell center.

Kingdom of mushrooms
These organisms have the characteristics of animals and plants. Their cell wall contains chitin, which accumulates glycogen. Some fungi have multiple nuclei in their cells. These organisms, like plants, are immobile.