Thinking about this, I think the reason could be what would happen to apps you remove from your home screen if you've hidden the app library?
I guess Apple could remove the option to remove apps from your home screen if the App Library is disabled, but then that introduces two distinct ways of managing apps (app library enabled: you can delete apps or remove from home screen; app library disabled: You can only delete apps), not to mention what happens to apps that aren't on your home screen if you disable app library (do they just dump them all onto your home screen?)
Of course, all of this would be possible, but it is a bit more complex than simply hiding the page and Apple are always keen to keep things simple...