Well, here's why:
(1.) The "infrastructure" is already there: all you'd need to do is add the dynamic node paths.
(2.) The universe is dynamically modeled and everything is moving... For an awesome but completely unnecessary level of realism, you can set the date to the given date in the FS universe, and all the celestial bodies in question will theoretically be where they'd supposed to be in reality.
(3.) If the given campaign requires a new node to be generated, ideally all you'd have to do is plug in a new node path.
(4.) For CBanis, everything as already "prettily" rendered and what have you. Getting that level of realism in Blender... or some other customizable viewing setting already available in Celestia would be much, much more involved.
I think that's a reasonable argument.
Oh, and here's what Mars should look right at about this time...

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