Question

a battery gets discharged why

Answer 1

Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf life of batteries and causes them to initially have less than a full charge when actually put to use.

Answer 2

Chemical energy gets converted into Electrical energy and vice versa.
If energy is utilised while conversation, battery will automatically drain out