Why is my food bar not going down?
You are in Creative mode or on Peaceful difficulty
Creative Mode
Creative mode strips away the survival aspects of Minecraft and allows players to easily create and destroy structures.
Minecraft Wiki - Creative Mode
Peaceful Difficulty
The Food Bar never depletes and players cannot eat anything except Golden Apples, unless the player has switched to Peaceful when their Food Bar was below the maximum.
Minecraft Wiki - Peaceful Difficulty