What you're saying would also imply that everything is already there in terms of stamps. Stamps take time to make, it takes time to prep everything to be done, it takes time to do the pressing. All this time needs to be paid for. Also that's assuming that it's all just a simple wet pressed stamp with no hand tooling involved. It's not rocket science, but that's like asking why said painting is expensive. After all we are just pushing colorful wet stuff across a semi-rough surface.
I like to think of myself as being an honest person, so I feel perfectly justified in saying that if I'm selling a product to a consumer it's not for my health, I'm doing it for profit and therefore I want my time and work to get a fair price.
I may not charge 90 dollars for a simple stamped sketchbook cover, but I would charge a price that is competitive and that I feel pays for the time and materials and labor that I put into a product. If it takes me a day to make an item then I need to be able to charge a good enough price to justify that time. Same reason any other skilled laborer or artist charges premium prices for their work.