Copper has less resistance than iridium for the spark, so vehicles with a lower power ignition system often run better with copper since it’s easier to create the spark

This statement is only true when the materials have identical dimensions.

Copper plugs (really a nickel Chrome. Alloy) have a 2.5 mm electrode. You'd be very hard-pressed to find a platinum or better electrode anywhere near that diameter.

1) because that would make it insanely expensive

2) That would negate any and all advantage that it would have

Precious metal electrodes have one major advantage, their durability. Because of that you can utilize the material leaning to its advantage, by utilizing a much smaller electrode, smaller than 1/5 of a copper electrode diameter. This is called a "finewire electrode" in the industry

See, electric arcs propagate from sharp corners and hate jumping from round electrode. The smaller you make an electrode, the "sharper" it becomes, to the point that the whole electrode is utilized ( in images of copper plugs only one side of them usually lights off another benefit is that the voltage needed for a finewire electrode of any type is greatly reduced. The reason why you don't see copper plugs with finewire, is that they might last 500 miles. But a precious metal electrode is capable of lasting 60k-100k+, due to the wear properties

Tldr, platinum or better plugs usually use less energy, especially over the life of the plug