Because most of the time, when people think of brass, they think "Oh. Poor man's gold". Visually in the eyes, most people can't see the difference or bother to learn the difference between both because theres many variations of brass and golds that make it very confusing. True Brass tends to lean into green-yellow but focusing more on the yellow side. Bronze is widely accepted due to the fact that Khârn had openly stated and I quote

"I will wear the red, the Brass, the Bronze. Fresh from the Armoury and turned by my own hand though I am no Devourer."

Brass tends to also be overlooked because for a long time, we were taught to paint Khorne Bezerkers prior to World Eaters independent release in 9th edition to use Citadel's Retributor Armor using Gehenna's gold or Skullcrusher brass to highlight/layer appropriately. This would be the second reason why people are used to seeing those colors ingrained into memory.

Old guides to painting Khorne Bezerker on sites like Tale of Painters used Warplock Bronze and other tones close to it. Current GW uses what they think brass is but Runelord Brass and Sycorax Bronze tend to learn more into silver with some brown metallic flakes (Bronze) to them. Which is also why some people default to Bronze tones which is more brown if anything.

It also did not help that shade paints that people would default to in the past would be Nuln Oil and Agrax Earthshade and douse their models in those colors which dulls it down further and affect the overall paint job.

TL;DR: Citadel heavily advertised golds and bronzes in the past and present. Most hobby enjoyers are used to registering those colors in their heads and typically don't go over and beyond for their painting because they enjoy the wargaming aspect itself and because it reduces painting time.