Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

Because they somehow don't consider that someone could have copied the books or discs before reselling them, but it immediately comes to their mind with digital copies riddled with DRMs

Because lawyers, accountants and MBAs making policy for things they don't understand.

The legalese in the US (which might as well be everywhere as you need to have compatible copyright with the US to have a trade deal with the US, and your country is in trouble if it doesn't have a trade deal with the US) is basically that:

• If you buy a physical copy, you've become the owner of that one copy of the IP contained within. As the owner of that copy, you can do stuff with it like read it, display it, destroy it, or sell it on to someone else thanks to the First Sale Doctrine (but you can't do other things like copying it, unless it's a DVD as there's a specific exemption for the copy your DVD has to make to RAM in order to decode the DVD). There's nothing the copyright holder of the original can do to stop you exercising these rights.
• If you buy a digital 'copy', you've not bought a copy, you've bought a licence to use one of the publisher's copies that they've given you permission to have on your device(s). They'll also have given you permission to do things like read it if it's an ebook or play it if it's a video game, but as it's their copy, not yours, you don't automatically get rights to do anything they've not explicitly permitted you to, and it's not in their interests to permit you to sell it on unless they think you'll pay enough more for a resaleable copy that it covers a potential future lost sale.

I'm sure plenty of publishers would love for the second set of rules to apply to things like books, and from a quick googling, it seems like occasionally academic textbooks have included a licence agreement instead of you actually owning the physical book, but I imagine that most publishers are concerned about bad PR from attempting this with a hit novel and also don't want to be accused of fraud for having their not-a-book-just-a-licence on the shelf next to regular books and thereby tricking consumers into thinking they were buying a regular book. EA attempted to double-dip over a decade ago with Battlefield 3, which included a copy of the game (with regular First Sale Doctrine rights) and a licence key for the online pass (which wasn't transferrable) and got bad press because of it. Newer PC games often come as a key in a box with no disk or a disk that only runs a web installer, so you've not got a copy of the game to claim you've bought and obviously only have a licence, and this seems to have caused less upset. This wouldn't work with a book, though, as you have to fill in the pages at the printing factory, and can't magically do it only after the user's got it home.

Pretty sure they tried to prevent the sharing and reselling of CDs at one point.

bourgeoisie lobby in the State

“Reasons”

The theory probably revolves around there not being a transfer of the file as much as a copy paste of it.

Physical media can be copied but you need blank physical media to copy it to, in a purely digital eco system both the buyer and seller end up being able to have copies of the sold product, which effectively treats the seller as if they are a vendor of the product being sold.

Basically in a world built on copyright law, being able to buy and sell digital media the same as physical media looks a lot like someone scalping the original product to cut into the maker's bottom line. Megacorps eating it is pretty dope but it significantly diminishes an individual developer's ability to profit off their own work as well, especially when software development already encourages so much copy pasting to make software that should be working into software that is working.