Because it's a metaphysical plane of existence. Is the light there really bouncing photons or is it a conception of divine radiance? Not saying you couldn't, just a thought I had.
I guess I just don't understand why the light would strike your eyes in a way that lets you see, but wouldn't strike a camera in a way that lets the camera "see."
The first part was already quoted and all it says is that the planes are in metaphysical relation, not literal spatial relation. That says nothing whatever about whether the metaphysical relationship is correct, incorrect, or a matter of faith either way.
The second is, frankly, worse--because it pretty clearly indicates, at least to my read, that this IS the cosmology, unless and until you the DM decide that it's something else. In other words, by even having it as written this way, it's claiming that the model
is in fact correct.
Of course, had I my druthers, the Great Wheel would be just one cosmology that got
just enough page space to give you a loose idea of each of the planes. And then you'd have three to five
additional cosmologies that each got comparable detail.
Instead, from what I can tell, the DMG spends
at most maybe two pages sprinkled throughout the text, mentioning that you can do things differently if you really really really want to, and then 50+ pages going over crazy excessive detail about the specifics of the (5e-remodeled) Great Wheel.
It's not hard to see which of these things is meant to be understood as the correct way to do things, and which is "well, if you want to, I guess."
Particularly in the context of much of the rest of at least the original 5.0 DMG, which is so aggressively wishy-washy "you can do whatever you want! You can do X, or NOT do X, YOU decide!"
I had really been hoping for a very specific, point-blank, "The Great Wheel is just a theory.
A GAME THEORY" Ahem. More seriously, something like: "The Great Wheel is one competing theory of existence, with benefits and flaws like any other. Whether that theory is, in fact, actually
correct about the planes is not a settled question. Perhaps your campaign has some of these planes but not all of them--and the theorized existence of the other alleged planes is just scholars far too pleased with their perfectly-symmetrical cosmos. Perhaps there are additional planes that definitely exist, but which the Great Wheel has no place for; in 2014, we revised the Great Wheel to add planes that had been developed for the World Axis cosmology, for example. Perhaps there is only one 'upper' plane and one 'lower' plane, with all others being merely territories in those planes, where Law and Chaos serve as the two 'gates' between them. All of these are valid choices, though you may need to think carefully about the consequences of a given choice, as (for example) the Monster Manual is written expecting the cosmology of the Great Wheel."