Most folks who attend knife shows don't buy knives. They are there to see, handle and enjoy the knives of makers whose work they may have only seen in photos, and may never be able to afford. Frankly, I think they deserve to get value for their price of admission, so leaving sold knives on the table allows them to experience those knives they wouldn't otherwise get to see. Heck, the only way you will ever see knives from some makers is if they leave a few sold knives on the table. I remember a few years back when Buster Warenski displayed his King Tut Dagger at the Blade Show. That knife was sold long before it was made, but I certainly deemed it a pleasure to be able to see it and admire the amazing craftsmanship that created it.
And seeing is only half the fun. Holding a fine knife in your hand is the ONLY way you can come to appreciate what isn't in the pix. Balance and ergonomics are not visible in pix, so getting your hand around one is how you can best come to appreciate what can never be experienced outside of a good custom knife.
Finally, having an array of my work on the table allows me to gauge what I'm doing right or wrong. If one knife gets ignored and another attracts a lot of attention, guess what I'm likely to build more of? I view my grasp of user interests to be an important part of my design efforts. Since I don't make any two knives exactly the same, I need this feedback to keep me moving in a useful direction. If a new design disappears off my table in the first hour I have no way of knowing if the design was good or the buyer just had unusual tastes.
I can't think of a better way for knifemaking to advance both in quality of makers and quantity of buyers than to expose the maximum number of people to the maximum number of custom knives. A knife show is both a commercial undertaking and a museum, to be enjoyed by rich and poor alike. It is also how new knifemakers learn what they might want to emulate in their own work. How can I answer the often asked, "how did you do that" question if the knife isn't there? Since that is fully half the fun for me in a knifeshow, I'd surely miss it.
That's my take anyway...