The only people who like owning physical books are either light readers or people who haven't moved homes.
I read a lot, and I have moved a lot. I still prefer physical books. We just bought a house last year, so I'm looking forward to not having to move again any time soon, but to me it was worth the pain. I get eBooks on the cheap when I can for traveling, but if I'm home I just can't do eBooks, there's something about the physical book in my hands.
Then again, I've got nothing on my brother-in-law or his dad. My BIL had to get a separate U-Haul for his books when they just moved last month, and his dad has converted all of his kid's old rooms into libraries. Then again, he is an actual published author, so that might have something to do with it.
Lowest averages I saw were around $6.10. I must not have refreshed at the right time to see $5.something.
I'm not sure where you're seeing decent used books at those prices. Used book stores that I've been to seem to act like used books are made of gold, except of course when you want to trade some in. Used books at thrift shops and garage sales can be decently priced, true, but there you're basically ending up with whatever you find.
That said, yes, ebook bundles have higher averages than game bundles. This is true for the Humble ebook bundles and for those from others like Storybundle. Ebook buyers are different from game buyers in their willingness to spend.
We have a chain of Half Price Books which sells at half the cover price, sometimes lower. Most books are 60-70% cheaper than cover. Then there are some smaller used bookstores that I absolutely love and know the owners at. Kzoo Books in Kalamazoo Michigan is probably my favorite store, but since I live in Chicago it's hard to get up there. Of course the bigger retailers of used books are more expensive.
I'm not really sure of the economics behind eBooks, as they're a completely different commodity than video games, but I would imagine at those prices, someone's being greedy.
How many marketplaces can you even buy eBooks from though? That might have something to do with it.

k
I understand the claimed economics, I actually had dreams of being a published author once upon a time, but I never felt like my books were good enough for other people to read. The publisher gets the biggest cut by a large amount, authors can be lucky to get 10% of sales unless they are particularly famous or self-publish. Editor takes a pretty huge chunk, and then marketing. The distributor adds a small markup, typically 10% or so. I think the main problem is in amount sold. Editing is a one off expense, so the more copies you sell the "cheaper" the editing cost was. They also claim that having to format the eBooks to work on all the different devices adds to the development cost, but I call bull on that. Once you do it once, it works the same for all books in that format. Of course, if they didn't need DRM the books could just be PDFs which wouldn't require any special formatting for devices.