Otherwise steel lines will preform for many years. Plain copper tubing will not take the pressure of brake fluid.

Nicopp Nickel Copper Brake Fuel Transmission Line Tubing Coil 3 8 X 100 39
Copper brake lines vs steel. The trouble is that they dont fail slowly like steel they fail suddenly. If this material fades or is scratched the plain steel line underneath is exposed and will rust. Cunifer copper nickel brake line is corrosion resistant on the outside. Copper and four copper alloys also out performed the double wrapped steel tubing in the tests. The report revealed that steel brake tubing was highly erratic after 4 6 years in service. This means that all the road salt and grime you pick up in the winter is ineffective at rusting the outside of the tube.
Catastrophically is the technical term. One thing to remember brake fluid dot 3 4 will absorb moisture most reference material claims between 1 2 per year. There is no saturation point in therory it will absorb all the water you can add to it. Theres a copper coating on the inside of the tubing too. This will double flare and bubble iso or single flare very easily. Theres a lot more copper in a steel brake system than just your banjo washers.
Moss may sell copper brake lines but ill bet they also sell that stuff that looks like copper but is far stronger and more resilient. Stainless tubing is harder to bend and double flare. It will be difficult with cheaper hand held tools. Copper brake lines are illegal in many countries as far as i understand. Cunifer copper nickel brake line is corrosion resistant on the. These come bare or coated in polymer wrap for corrosion protection.
The exposed conifer copper nickel brake line is still 887 copper. Well as the video above mentions your standard steel brake line isnt exactly as simple as it sounds. It also identified copper nickel alloy c70600 tube as outstandingly superior to conventional steel brake line tubing in laboratory salt spray exposure burst tests. The moisture in the brake fluid rust metallic parts and will corrode aluminum. Wrapped brazed steel is the norm for brake lines. Normal steel brake line is coated in a corrosion resistant material.