That's not quite the same thing. When I buy a gun, I can just buy a holster for it from JMCK or whoever. Sure , shipping, taxes and customs fees are annoying and add to the cost, but it's still a one way process resulting in a $100 accessory for a $500 pistol. And I can go shoot with my pistol while I wait for the holster. But there are so many knives out there that most of the time, sheath manufacturers do not have a mold for that particular knife. Which means I would have to send my knife to the sheath maker, who will be somewhere abroad, probably overseas in the US. I don't have access to my knife while the sheath is being made, and now we're talking about a $150 accessory to a $50 knife. (All numbers in this post are of course off the top of my head approximations.) And should I really, really like a certain knife, perhaps something on the expensive side (so I can't just buy a second one for the express purpose of sending that one to sheath makers)? I can't tell off the top of my head exactly how many 1911 holsters I own. Buying multiple sheaths would be much more aggravating than buying multiple holsters.
While the sheath that comes with a knife might not be absolutely perfect, for me at least it is important that the knife comes with a sheath that is usable for me. I've decided not to buy a whole bunch of knives because of the sorry ass sheath that comes with it.