Why have botanists for a long time believed that mushrooms are plants?

Mushrooms are an independent kingdom of organisms that differ significantly from plants and animals. However, fungi have traits of both plants and animals.
Mushrooms are similar to plants in their immobile way of life, causing this similarity to confuse botanists.
Fungi have a cell wall and a vacuole, just like plants.
Both kingdoms are capable of unlimited growth and reproduction by spores.
The way of feeding both those and other organisms is the absorption of nutrients.
However, fungi are heterotrophs and do not have chloroplasts, which does not allow them to be attributed to the plant kingdom.