Because they're desperately trying to protect the sales of hardcover books, and, in part, trade paperbacks. They don't want to risk cannibalizing sales.
Consumers look at ebooks like another variation of the same product. Publishers don't. To them, it's an entirely separate entity. To them, the question you're asking could be (very roughly) translated to "I don't understand why concert tickets aren't the same price as a CD. It's the same music."
It's a bit backwards, and the same kind of thinking that made the MPAA consider the VCR a threat to movie theaters and the MLB think that nobody would buy tickets to the ballpark if games were broadcast for free on TV.
But I can understand why they're doing it. An enormous part of their legacy business model is wrapped up in hardcover book sales.
As for production costs, it would only be paper, printing, and distribution. Nobody ever thinks about pre-production costs or employee costs in professional editors, typesetters, and jacket design. That's a lot of dough that they have to lay out that still needs to be paid out whether we're talking ink or bits.
If the book is unencumbered by DRM, then yes, I agree it'd be fair. Otherwise there's a certain hit to the utility of an ebook versus a printed book and the price needs to reflect that.
I can't lend it to a friend (let's face it, Amazon's current "lending" policy is just lip service). I can't print out a page or two of a cookbook so I don't have to spill sauce all over my fancy e-reader or iPad. The DRM nullifies any first sale rights I have, so I can't gift it, donate it, or trade it to the indie used book shop down the street from my house.
I personally thought that Amazon's original pricing at ten bucks a book reflected that pretty well, and allowed me a good tradeoff - I could pay $10 for the DRM restricted version, or $15-$20 for one with full rights. If I want something to read while on an airplane trip and forget about later, I'll buy the ebook. If I want a family heirloom, I'll buy the paper version.
What I've read from publishers is that the actual productions costs aren't that much. The fixed costs are still there, and some additional costs because now you have multiple formats.
I agree it would be fair. I don't agree that it is unethical. But I think it is counter productive. Attempts to price electronic items higher than the physical ones will only result in those items being pirated. They have not yet learned this lesson because they clearly think that the higher price will lead to more sales of the printed items.
I guess every industry needs to learn this lesson.
On the upside, the more they get addicted to the revenue from the electronic versions, the more strategic it will become, and once it is sufficiently strategic to them-- rather than the experiment I think they see it as now-- they will start having to seriously compete.
This will change the market dramatically, and most effectively.
The alternative- forcing their prices to be lower will only result in them removing availability (e.g. they'll stop publishing ebooks.)
Discriminating certain distribution channels might be good for profits, but I think it's clearly unethical.