Why should the wealth be of a growing nature?
A wealthy person gives some part of his/her wealth to the poor and needy and they fulfill their needs with it. The objective here is to develop sympathy in the society and to ensure that the society isn’t plagued by economic crises. In light of this, the amount of Zakaat is very less so that the person giving it doesn’t become needy himself.
If Zakaat were fixed on non-growing wealth, then it is possible that the person giving Zakaat today might, some years down the line, become needy of it himself. The system of Zakaat is to do away with economic crises and not precipitate them.
Thus, Zakaat is compulsory on wealth of a growing nature. When the owner of the wealthy removes some money from it, then as the wealth is of a growing nature, it will be made up soon.