Well, I have access in Files to a lot of content from my apps - that’s a decision of the app creator to not use this and keep the content created in the locked area of the app.
For example, the apps from Omni do this, as do obsidian, Linea…
Obviously the blame lies on Apple for locking away your device's contents from you. Developers should not be able to have more control over what you can access on your device than you do. Even if they make bad choices (like making accessing the files hard) it should be you who has the final say, not them.
Apple making it possible for developers to make bad choices and go against users' control over their own devices is to blame.
That's easy, just store all the user-generated data in an encrypted file. You might be able to copy the blob but the vast majority of users won't be able to extract the files within.
For example, the apps from Omni do this, as do obsidian, Linea…
Let’s assign the blame where it should be here.