Why are mushrooms not classified as plants or animals?

Mushrooms, like plants, are motionless, grow throughout life and multiply by spores. But, they also have signs of animals – they are heterotrophs, that is, they are not capable, like plants, of photosynthesis. In their cells, the cell wall is made of chitin, and the metabolic product is urea. Therefore, they can not be attributed to either plants or animals. In addition, their body consists of filaments and is called mycelium, and fruiting bodies are formed to form spores. Therefore, the mushrooms were allocated to a separate kingdom.