Just in case it's not clear the reason is simple. It's not unlike you buying an used car from somewhere and then asking a third party to warranty it without inspection.
Let's just say it is unlikely to happen. In fact such a warranty should push the price back to a new laptop with the factory warranty!
I am thinking of buying a refurbished Asus through Best Buy. The warranty is only 90 days though, and Best Buy doesn't offer an extended warranty on refurbished computers. I talked to Asus directly, but the only offer an extended warranty through the third party store. Is there another place where I could purchase an extended warranty for refurbished computers?