Because they aren't all refurbs.
But aren't most refurbs? Also I don't think DK serials mean its a refurb. There are plenty of replacements out there which don't have DK serials and chipmunk's serial number lookup site is the only site which claims that DK is a refurb.
In Apple's parlance almost none of them are refurbs. They use that term strictly for retail items sold 'open box' on their site.
For service parts they are either totally new or new with some refurbed parts. But they call the latter remanufactured, not refurbs.
And can you tell by serial number whether its refurbished or remanufactured, since you say "most" and not all are remanufactured?
Not all brown box replacements are refurbs. Early in the product cycle, there aren't any refurbs because no products have been returned yet. So some new products are set aside to serve as replacements and shipped in brown boxes, but they are new. The later it gets in a product cycle, the greater the chance that brown boxes are refurbs.