The aliens are called "flats," not to be confused with a unit of housing. They consume light and other forms of energy. That is what feeds them. The reason they can only see us through cats' eyes is that cats' eyes are very sensitive to movement, and flats cannot distinguish us from static images like chairs and walls, etc. They can only detect us when we are moving but they lack optical sensors of their own. The flats' connection with cats is unclear, but they have been visiting earth since prehistoric times. The superstitious myth that cats are the devil's eyes has some basis in fact, though it is not certain whether the flats are ambivalent towards us.
On one hand, flats provide shelter from the heat to people all over the world, but on the other hand, they have yet to reach out to us in an overture that we can understand. They mimic us and often imitate or follow our movements via their attachment to the filtering of the sun's radiation. Most people fail to see the connection between solar radiation and the activity of flats. People falsely assume that our bodies absorb the light from the sun and that shadows merely mark the absence of light, but in fact, our bodies serve only to deflect portions of the solar radiation and thereby make the remaining rays more palatable to the flats. That is why they flock to our projections.
"Shadow owes its birth to light" - John Gay, British Poet (1685-1732).