Because it's so totally arbitrary. There's two ways to look at this, since we're talking legalities here - holistically, or technically.
Holistically (ie overall), this is a load of cods wallop for reasons that have already been gone into in this thread. Who cares if it's on the disk? The value proposition of the game you purchased doesn't change whether it's on the disk or not. Is the game worth $60? That's up to you to decide. The existence of some code or assets on the disk doesn't change that. The work of the people who may or may not have been making DLC for 5 years doesn't change that. These 50 (I just made that number up - the point is, it doesn't matter) people being hired specifically to do it, being taken from another team, being outsourced workers in India, being the core team who've been working tirelessly on DLC for a few years - none of this changes the value proposition that you're faced with when you decided whether or not to buy the game. When you buy a game, you aren't buying the whole fruits of labour of the development team between the last game and the current, nor an arbitrary period of time, nor anything else. You're buying the offering in front of you. So in that sense, the idea of DLC being "illegal" if it's on the disk is utterly laughable, like complaining that Malware Bytes Pro version is already installed, and all you're paying for is the license code and suggesting it be illegal as a result.
The other way is technically - you've paid for the content of the disk and you should therefore have absolute access to what's on the disk, in the same way you should be allowed to rip a CD to your hard drive because you should have full access to that CD. And you do have full access to the disk. You have full access to the compiled code that sits in 1's and 0's on the disk, and you can do anything you want with it. Except you can't, because you don't know how. You don't know how to use that data outside of a PS4, and even if you did, you wouldn't know how to unlock the hidden stuff. Maybe it's not even possible - but you didn't buy the source code, you bought the compiled code, that's what's on the disk and that's what you have to play with. In that sense, what you have is akin to buying a CD but not having any CD ripping software. Well, that ain't Universal Records fault.
The only sense in which it could be argued to be illegal is in an entitled "It should be illegal because I want it for free" way. Which isn't a very good basis for legal argument.
In short, you didn't pay for that content. It doesn't matter if it's on the disk, on Bungie's servers or etched into stone tablets in Mount Sinai - if it isn't playable, it wasn't part of the value judgement. If it were, you'd be able to play it. But you can't.