Don-
I"m sure you would want to do everything you could to appear to someone after you die in order to reassure them that life goes on.
As to why such occurrences are so rare, it's entirely possible that the reason is much the same as why credible UFO sightings are also relatively rare (especially these days when almost everyone has a camera phone!).
Or to put it another way, if UFOs and aliens were real, there would be no reason why they would appear only to a handful of people. And if we agree that they actually have appeared, we then must acknowledge that they intended to be seen.
And therefore the next question is, if they intended to be seen, there's no logical reason why they wouldn't want more people to see them on a more frequent basis.
So let's take your Leonard story. If Jeff was able to appear to Leonard after his death, and actually carry on a conversation (including driving a truck while they talked!), there also is no logical reason why others could not do the same thing.
After all, there was nothing especially profound that Jeff had to communicate to Leonard. No earthshaking revelation. All in all a pretty mundane conversation other than the obvious fact that one of the men was dead.....

So I'm left with only one logical conclusion: Neither UFO/alien sightings nor ADCs are objectively real.
We humans are capable of tremendous self deception, including highly credible people like airline pilots, military observers, and ordinary folks like Leonard.
R