Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

There's a few things about this

1. Many times you don't own the digital good, you subscribe to it. No I'm not joking, that's why services can usually take it away at any time. You normally own "a licence to play it on a single PC" or similar.
2. This isnt apples to oranges per se. Selling digital goods is fine, it's copying it. Similar to how photocopying a book and selling it would not be okay.
3. It's important to note there is a narrative push by companies too. They spend lots of money putting videos on every DVD saying "downloading is stealing" because if society thinks piracy and stealing is the same, it helps them litigate and make more money.
4. Your idea of a lost sale is a hard one, from a media company point of view, it's about making money. So if you can make people believe "a download is a lost sale" or "sharing a digital file is a lot sale" etc, then you can use that to sue individuals, isps, sharing sites, search engines etc and make more and more money while also having more power over your product.

Because the book and disc guys couldn't figure out a way to stop you back then.

Nowadays college books have one time codes for tests, and games will sometimes have codes included for inportant unlocks to force used purchasers to pay up.

There is no difference, they just want you to believe there is a difference.

Cos if you believe it then they can make more money.

The point: GREED that's the difference.

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

One thing to keep in mind that may be relevant: copies of non-digital things are different than digital copies.

Digital (meant here as bit-for-bit) copies are effectively impossible with analog media. If I copy a book (the whole book, its layout, etc., and not just the linguistic content), it will ultimately look like a copy, and each successive copy from that copy will look worse. This is of course true with forms of tape media and a lot of others. But it isn't true of digital media, where I could share a bit-for-bit copy of data that is absolutely identical to the original.

If it sounds like an infinite money glitch on the digital side, that's because it is. The only catch is that people have to own equipment to interpret the bits. Realistically, any form of digital media is just a record of how to set the bits on their own hardware.

Crucially: if people could resell those perfect digital copies, then there would be no market for the company which created it originally. It all comes down to the fact that companies no longer have to worry about generational differences between copies, and as a result, they're already using this "infinite money glitch" and just paying for distribution. That market goes away if people can resell digital copies, because they can also just make new copies on their own.

bourgeoisie lobby in the State

Usually, when the question is "why is copyright restrictive in these evil and/or dumb ways" the safe bet is that the the answer is "Disney."