Only because there's no universally standard frame of reference. In theory scientists could all agree on some arbitrarily determined points X, Y, and Z... but it just wouldn't be as obvious as the ground you're standing on. Up and down only makes some sense on a small regional scale for if two people would stand on either pole and one pointed up, the other would have to point down to point in the same direction.
When you go into a solar system (or into a larger system such as a galaxy) you need to use a different set of criteria based on some readily used criteria. In our solar system, or in most spiral galaxies, this can be 'Spinward, anti-spinward,Center-of-Mass Inward, Center-of-Mass Outward, Right-Hand-Rule Upward and Right-Hand-Rule Downward.'
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