Retrieval of UAPs

Please help me understand why NHI able to cross Galaxies should crash / run out of fuel / get shot down on earth? And even if it happens, why wouldn't the other NHI make sure to retrieve their spaceship themselves to avoid detection by humans?

One explanation I would have is, that the NHI is doing it with a purpose. Either as a scientific test object, or to help us advance our technology....

Other explanation would be that they got shot down by an hostile other NHI group. But then I would still expect them to do a retrieval operation for their own spacecraft and not allow us humans tp grab their spacecrafr.