Yes, they absolutely are. The memo about supply and demand seems to have been mysteriously lost in the conversation about digital products. An infinite supply (which you get with digital content) should mean lower prices. Yet they're going up instead.
Perhaps this is kind of a tangent, but it has been my experience that in general, we seem to have some sort of sense when WE are the ones producing an item that "the economic value of an item should be tied to the amount of labor that went into producing it, including the sunk cost labor invested in developing the skill" and yet when we are purchasing items tend to argue that "the economic value of an item should be tied to the materials involved in producing it."
In other words, we're always interested in emphasizing the side of the equation that benefits us. I always look at something like a hand-crocheted blanket and think of the time and skill that went into it... it's many, many hours. And yet when purchasing the blanket, most people want to pay only for the value of the skein of yarn that went into it.
There is labor involved in producing a book (even a PDF). Since Gutenberg, the labor and skill involved in imprinting ink on paper and binding the paper together has been going down. But one could argue that the amount of skill required to arrange the words and letters and art together into a product others want to purchase has been going up.
So even though PDFs allow for near-infinite supply of the finished physical product analog (the PDF file), that's only
part of the production equation where cost to produce has gone to zero. You don't have to pay for the paper and the binding and the printing press with a PDF. You DO still have to pay for the skill and labor of the author that wrote the book (and the editor, and artists, and layout artists, etc.)
So while you may have an "infinite supply" of one book, you don't have an "infinite supply of all possible books." And with total units sold being lower as market fragmentation increases, and value of the skill involved in production going up, the cost to produce one "template" from which your "infinite supply of copies of this particular book" can be made is increasing while the number of people demanding one of those infinite books is decreasing.
Reducing the cost of physical production to zero has helped shield us from that reality, but if it now costs more to create a creative work than it did in the past and fewer people are buying it, that tells me the price is likely to go UP.
I used to work in book publishing. It's a nightmare all around. They understand that digital means infinite supply because they require libraries only check out a book so many times before the license expires and they purchase a new copy. So clearly they get it. But that's seemingly never applied to book sales. Unless there's a bundle where you get like 30 books for $10.
This is a separate part of the equation entirely. Book publishers want to enforce artificial scarcity in a digital world, which is why they want licensing to eliminate the right of first sale* (or the non-US equivalent in other countries). But that gets into a whole different argument which is only relevant to the original post ("why are ebooks getting so expensive") in the rather trite and boring answer "because publishers are (and always have been) trying to maximize profits and will charge as much as the market will bear." (The means by which they manipulate what the market will bear are tangentially relevant to the conversation, but the WHY is more important).
* This is relevant because in the past, a physical book publisher's competition was not just "other books" it was "copies of the same book I may have previously published available on the used book market." This is less an issue in a "licensed-only" world where I cannot obtain the licensed copy of an e-book someone else has decided they are finished using.