There's been many UFO sighting on earth over the years, seeing we scan for celestial bodies that might hit earth?

Why haven't we detected them coming to earth, if they can't stop us from seeing them once here on earth?

It doesn't make sense, why come in stealth then give it up once here?
Update: @ReductioAdAstronomicus ·with all due respect, many military and law enforcement personnel have come into contact with these UFOs. My point is that what they must of seen must not have come from outer space but been man made.
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