The concept of owning the copy is not strictly correct, as @doctorlighthouse advised above, you really only own a license to use the product copy in the way that the agreement you have consented to allows until the copyright on that product ceases at which time you are free to do with it as you wish. Mostly you only ever buy a license to use the digital product, this license is generally for your lifetime. The ownership of the actual product remains with the copyright owner. When you purchase the license you are given certain rights, these may include the ability to re-sell, gift, copy, play, view and so on but you never own the original product. When you purchase from ITunes, Amazon, Microsoft, Corel or any other producer/on-seller of digital products part of the EULA (End User License Agreement) you make is about your usage of the material you have purchased the license for.
Why is a hard copy of a book different? Well it isn’t, you don’t always have the right to sell a book you have purchased in any way you choose. If, for instance, the book is only intended to be sold in the Australian market and it is stipulated in the book then you are bound by that when you purchase it and therefore are not entitled to sell that book outside of Australia. You may very well sell it outside of Australia but legally you have broken the contract. There are normally other limitations on the use of your book some of them being, you cannot reproduce it in any other format/s, you cannot legally make multiple copies and sell them, though to do this illegally would probably be expensive to do so it is not something someone would normally consider doing.
The ability for someone to sell a digital product multiple times without proper recompense to the owner of the work is very easy to achieve. Someone may purchase/pirate one licensed copy and sell 10, 100, 1000, or more copies to others with very little effort or expense, in so doing they will have stolen the income that the owner is legally entitled to. So they, the Copyright owners, generally only allow the use of the digital product in their license not the selling or copying on to others.
Some digital products eg Adobe Acrobat (full product not the reader) do allow you to sell/gift it to others but when you do sell/gift it part of your license obligation may be as in this example advise Adobe who the new user is, to completely remove the product from your device/s and to make sure the purchaser/giftee understands and agrees that they have a license to use the product only in the way the license allows, and if they don’t agree then they cannot use the product.
Some Licenses are more free than others and particularly those under GNU GPL (GNU General Public License) licenses allow free and unfettered copying, modification and selling. For more about free and not so free licenses see: