Probably cause it's just dye... not a conditioner or it doesn't include some sort of oil base to be shiny.
Plus you're thinking about anime characters, who don't have to deal with real sunlight, backlighting, shadows, fading, translucency, etc.
Real people hair just doesn't have the volume (ie solid blocks in some cases) or the stiffness (weird single hairs unique to certain characters) to pull it off normally.
There are wigs, and a whole heck of a lot of hair spray / gel to keep it up. Cosplay.com has "some" good examples where they really put effort into making it look good.