World governments just might be shooting them down.
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That point is the most unlikely IMO. It would be extremely hard to keep that many people from leaking/talking about such spectacular events. If you’ve ever worked in an organization/bureaucracy you know how hard it is to keep secrets for long periods of time. Stuff gets out eventually.
With respect to your other points, it is entirely possible that our notions of what alien spacecraft are and how they operate may be completely wrong. Human experience and bias often color how we perceive things. If they exist, such spacecraft may not be of a form we are familiar with.
Never mind the leaked documents that DID get out.