It probably depends on how you define “vegetable.” Biologically, anything with seeds is a fruit, while a “vegetable” is a traditional word meaning the edible part of a plant. It is not a scientific definition, because it encompasses flower buds (broccoli) roots (carrots) fruits (tomatoes) leaves (kale), etc. Biologically tomatoes, cucumbers, peppers, avocados, etc are fruits, although in culinary use they are vegetables. Avocados, for instance grow on trees. I would guess that more true “vegetables” (that are not fruits in any sense) don’t grow on trees because trees are generally fibrous and tough, unlikely to be palatable to humans. More here