Can copper be used for fuel line?

Copper will work harden, not just with vibration but with heat cycles as well. That is why we replace the pure copper spark plug washers on aircraft engines (or anneal them) each 100 hours.
Copper fuel line is more expensive, because.....copper is more expensive.
Ever see a car on fire in the middle of the road? Use whatever you want, it's your car. I myself have plenty of copper tubing available, but won't use it. Aircraft fuel lines are either stainless steel, aluminum, or braided steel teflon. Copper is only allowed on primer lines.....which frequently break! I've never seen a steel or aluminum line break except in an accident. I rest my case.