It’s one of 2 things. Sometimes buyers…forget. They got what they wanted and left the site, assuming everything will work out fine. I already paid, didn’t I? What more do you want! Is probably what they are thinking. I’m not going to leave an icky review! Whatever. I got better things to do. Now yes, not all buyers who forget think like this…but these people still exist.
Another is like @wordsfire said. Resellers waiting for the original buyer to rate them before doing the same. Or even not doing so at all, because sometimes (for orders involving art/audio/video) it shows up on the sellers page after they accept and leave a review. And the original buyer might see and realize the reseller is a fraud. So instead of even risking that scenario happening by accident, they don’t even accept/leave a review. Which leaves US to wait the excruciating 3 (or during the holidays 7) days for the order to auto complete.
(Side tangent. I found a hack for fellow writers when delivering orders to resellers.)
I’m dealing with one right now, but they think “Since it’s a writing gig no one will know”. But I’ve recently started adding screenshots to the orders of my writing projects to hinder these resellers from pulling a fast one. Yes, they can simply not leave reviews. But a lot of these resellers don’t think that far ahead and are just trying to rush and deliver because they are on a right deadline.