Those aliens capable of interstellar travel sure do seem to have a lot of mishaps when they hit Earth's atmosphere . . .
Our 20% Oxygen air is very corrosive to their structural materials from their transuranic "island of stability" structural elements.
Morover, their metabolism, based on a chlorine atmosphere and a copper blood "hemoglobin" does not do well here, and their first attempts at exploration and recon resulted in the damaged and intact vehicles we have come upon and from which we recovered cadavers.
They have since learned to cope with our atmosphere on their ships but still need a 20-80 chlorine-nitrogen atmosphere to survive. We have recently discovered that, so some of the recent ones have survived in our artificial Cl-N atmosphere chambers.
We have learned to communicate with UFO crash survivors and found they are friendly and have helped with many of our recent technological advances.
In addition, they happen to resemble kittens.
Terry, 230RN