Why do people use "Mahlzeit" as a greeting any random time of the day?

My colleagues used it at 6:00 where everyone is still half asleep at the computer at the office. They used it at 10:00 when no-one was eating, they used it at 16:00 when no-one was eating. It annoys me to no end when it's used outside food context. Why why why? Help me understand pls.

Edit: Thank you, good people, for all the replies. The history and context of how this came to be is making me more chill about the greeting, but it also feels good to see that even some Germans find it odd. This was very enlightening. Danke schööön!