Why wasn't another ship sent to destroy earth?

or for that matter why did they never send an expedition when the base was out of contact for 2000 years... The answer is likely the same to both questions. Might have to do with scope. our view is earth centric, their view might have earth as a minor detail lost in archives. or maybe 2000 years isn't that long in their view of time. Or maybe they are too busy dealing with some other situation

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