Prior to the advent of the 40% discount bookstores (Borders/Barnes & Noble) they always actually get cheaper over time.

Those stores used economies of scale to make the 40% discount (basically selling for cost) on blockbusters to drive business into the store, with the hope that people would also pick up higher margin items (toys, coffee, etc).

But the more traditional bookstore model was to stock the blockbuster hardcovers at full price, then move them onto discount shelves as time from release went down, until they had to return them to the publisher right before the soft cover release date neared and the books became a liability (as the people that were willing to pay the premium for early access had already been exhausted).

There are still places that have heavily discounted hard covers. The entire business is called 'remainders' and the publishers sell giant warehouses full of hard cover books for pennies that they deem unsellable as they are too far from release date. Soft backs don't go through this process as they are even less valuable, the publishers pulp those, sometimes at a loss.

[edit] another more obvious way to think about this is, why do hardcovers and soft backs come out at different times? If it was just about the 'premium' product vs the lower end one, they'd be released at the same time. But they aren't. They are release about a year different.

The fact that paperbacks didn't come out until months after the hardcover came out? I mean, this is what my parents told me the deal with hardcover books was in the 1980s, but it's not like it's hard to figure it out.