Because they have a copy that is complete except that it is missing the centerfold.
Whether it "adds" $3K to the value, is another question. I think it would.
Or maybe you would be better off buying a complete coverless copy for $7K, removing the centerfold, and trying to "salvage" the coverless/incomplete remainders for $4K. That might work.
Sometimes the bigger factor is how well the parts "match" (page quality, cut, staple holes) the rest of the book that you have.
There are a lot of Golden Age books out there that are missing either a cover or missing the centerfold. It's a matter of finding and marrying the two. There is a rather healthy market out there for Golden Age parts. Sure the resulting book would get a CGC qualifed or restored label, but I doubt if the people looking for these kind of books are the type of collectors that are into slabbed books.
The reason this is CGC slabbed is to certify the authenticity of the pages.
Just this month, I bought a coverless & missing first wrap Superman #20, to "complete" a copy of the book I've had for years that was missing the centerfold.
ECFANMAN