Beckett's Graded card section

Why does Beckett list only rookie cards in their graded card section? Judging from ebay auctions, non-rookie insert cards of stars seem to go for more than the rookies in PSA 10. Any thoughts?
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Comments
(graded and traded) to justify the amount of time
necessary to determine values and also print these
values.
BTW: Cubby=Cub Fan