why does store engery not change?

A capacitor stores energy rather than charge because the electric field that is created between its two plates when a voltage is applied serves as the mechanism that allows it to do so.

Two conducting plates and an insulating substance known as a dielectric separate two capacitor plates.

Whenever charges of opposite polarity are separated, electrical energy is stored within the capacitor by the same charge present within it.

So a capacitor stores energy but not charge.