Why does a bird in your front yard fly away if you walk towards it ?
IMHO, these UFO's are not craft with intelligent life in them, the UFO's are the intelligent life.
We have life that lives underground. We have life that lives deep in the ocean where it's dark and cold. We have life on the surface. We have life in the air. We have life living in places we used to think impossible.
Why would we not have life living in the upper atmosphere and the edge of 'space' ?
I think Ivan Sanderson postulated something similar - that they represent some kind of life sharing the Earth with us that we just don’t recognize for what it is.