This conflates mathematical existence with actual instantiation. A 2gb integer might be definable, but until someone encodes a particular arrangement of bits and gives it context, it doesn’t exist in any practical sense. We don’t treat all future novels as "already written" just because their ASCII codes can be mapped to integers.
Sorry, but this is just complete numerological nonsense. All novels do not already exist. My proof is that if they already exist you will show me a novel that will come out a year from now today. The act of creation, of ordering words and other symbols in such a way that they convey a particular meaning is a non-trivial exercise to the point that we have created laws and reward structures for the people doing such organization. If we follow your reductive reasoning all media already exist. But they do not. The underlying principle here is the one of ordering, to take a chaotic or boring concept (say, an array of random or blank bits) and to impose order on it so that they take on meaning when used in combination with a suitable interpreter.
This kind of imposing or order is an act of lowering the entropy of the sample in a very specific way, parties that know the 'key' to the sample will be able to experience the sample in a way that parties without the key would not, to them the sample is still boring or random. Your reduction of the act of creation to picking a particular number is belying the fact that absolutely nobody that creates something is picking that number: the number is a carrier, it is not the ideas embedded in it. You could translate that novel (or textbook, or sound or video or any other medium) into other media, descriptive, literal or you could even completely transform it. And there would still be a relationship to the original creation, hence the concept of a 'derived work', which for your numbers example would utterly fail: you could not take that number outside of knowing its meaning and come up with any of these derivations without having the key to decode it.
This kind of reductive reasoning is not helpful, it merely attempts to flatten a whole pile of some of the most accomplished and positive contributions by humanity to the generation of interesting numbers. And it is so much more than that.
Besides all this, any kind of attempt to digitize an actual work of art, rather than just a simple text is going to be a lossy process. You are never going to be able to replicate the original to the point that you have created something that is equal. You may be able to get close but it won't be the same thing. More so for sculptures than for two dimensional art, less so for for instance audio where the replication gear is getting really good. But generation loss is a thing and if you re-create and re-digitize then after a surprisingly low number of such generations you will end up with noise.
Authors, sculptors, painters, even programmers and other creative people are so much more than interesting number generators, even if their works can be encoded or approximated numerically. That's flipping the encoding analogy on its head, the map really isn't the territory.
I said all novels already exist. That's different from claiming all novels have already been written.
The claim is that humans are not "creators" but generators, very much in the random number generator sense. We are interesting number generators.