Why do typical DC motors use brush contacts rather than just conductors?
You're not looking at what a brush actually has to do. Brushes slide, need to maintain contact, wear, and arc. So you need spring loading, low friction, resistant to welding and corrosion, and need to be soft and replaceable so they wear instead of the expensive, difficult to replace commutator ring, but not too soft or they won't last.
I also have a feeling you are imagining a bristled brush with copper bristles. Motor brushes do not look like that, at least carbon brushes don't. They look like block of graphite with a coil spring. I've never seen what a precious metal brush looks like in person.
EDIT: Searching around, precious metal brushes are probably more what you might have in mind when you asked your question. Little strips of metal cutouts to make fingers (not bristles) where the strip itself acts as the spring.