Because it was night. And I am not trying to be funny. If it did the tail behing the other plane thing, you could stay way north, and no one would “see” it, or literally see it.
I’m not buying it though.
I think it’s on the bottom of the Ocean.
Shadowing another plane to avoid radar detection is not done from behind but from directly above or below. To military ground radar, the two "stacked" aircraft appear as a single blip; to civil air traffic controllers, the blip will correspond to the transponder signal of the unwitting escort. In the process, the non-transponding ghost aircraft cruises along the regular airline route with its host, completely invisible.