It just isn't.
Copyright as it was originally concieved, and in every evolutionary step since then, has depended intimately on the specific limitations of physical media.
Computers are designed explicitly to allow information to transcend the limitations of physical media.
It's all about access restriction.
Access restriction is what makes information valuable. You won't pay for information you already have access to, even if it's very, very useful or desirable. Once you have it, buying access to it holds no value for you. If you lose access, but still want the info, you'll be ready to pay again. There is no difference in the amount a person will pay for -- that is to say, very literally, the value of -- a copy of information you don't want, and a copy of information you already have.
Access restriction is inherent in physical copies of information. Creating analog backups is imperfect and loses information slowly over time; creating copies similar to the originals is an industrial endeavor with not only significant costs, but physical traceability. If you buy a book, and you really want to, you can track it back to where it was printed and go there. You can only read a book if you're near it, you can only play a record you can touch (or command a robot to touch, whatever). If I make a million copies of something, I've created a million times the value I started with, because there's a million times more access to the work.
Access restriction is NOT inherent in digital copies of information. Identical copies can be created and destroyed indefinitely without the slightest loss or measurable cost, by whoever has the equipment to access them in the first place. A bit cannot be traced; any information you might use to differentiate one copy from another is inherently additional information, and can in turn be copied or removed. Anyone with access to the computer system of a person with a copy, also has access to that copy. Or any number of copies. If I make a million copies of a file, I have not created a million times the value I started with.
Computers force us to confront an interesting truth about information: an individual copy of information has zero value. The creation of that information has value, and access to that information has value, and THAT'S IT. The value of a physical copy lies in granting access to the possessor. If I can't see someone play music whenever I want, but I can get a recording, that recording has value. If I CAN listen to someone play music whenever want, wherever I want, just by waving my hand, a recording of them has no value to me. If a million people want access to the file, and I make them all pay first, only then is the file valuable.
Access control on digital information is essentially binary. If you give someone access, they have it, and if you take it away, they don't; and, immediately, EVERYONE with access to that party ALSO has access. DRM tries really hard to pretend that you can give and deny access to information simultaneously to the same party, but it's just a shadow play. It relies 100% on social factors to work (I don't want to break the law, I don't mind this business model, I'd rather spend the time to do something else than crack this or find someone who will). Because in the digital world, you can't grant and deny access to the same party simultaneously. Not really. You can limit access, but if the degree of access I want is the degree of access you're giving me, I've got it, full stop. And so does everyone who as access to ME.
The ONLY meaningful access restriction to digital works in the modern age is at the point of creation. The only reasonable business model is not to release a work to anyone until it is paid for, or not to CREATE it until it is paid for.
Interestingly, this is, in fact, how nearly all "Big Content" is created already. The budget for a movie doesn't come from future ticket sales, it comes from ticket sales from previous movies Big-name novelists get advances before they even start writing. When you buy in, you're not actually paying for the work that was created, that was ALREADY paid for. You're paying for the next thing.
The money that was used to create the movie Avatar came entirely from movie watchers. Every single dollar. Not directly, of course, but ultimately.
So why not directly? There is no practical or technical reason why not.
This is what terrifies the middlemen.