Why Do Pilots Use The Term Zulu Time Instead Of UTC?

Any pilot will tell you that aviation runs on its own language. The words and phrases transmitted over radio frequencies around the world every day are not informal shorthand or inherited habits. They are an engineered communication system, designed over decades to be understood clearly by any pilot or controller, regardless of native language, regardless of radio quality, and regardless of which country either party is in. Every term in that system exists for a reason, and most of those reasons trace back to a specific problem that needed solving.