Animals have to eat. Space aliens are animals so they would have to have food. The speed of light sounds fast but it still takes years and decades to go anywhere and if you don't know where you are going you could be in deep shit regardless of what kind of space alien you are. I doubt if they could survive on our planet without special equipment and I'm sure that they wouldn't come for a vacation. Is there any evidence that they are doing mining or eating us? Of course they could be robots but what is their purpose?FK, you attempt to limit other civilizations to the physical laws we've discovered. That's not reasonable. Our science-based technologies are around 300 years old. Think of the technological advances of the last 20 years. Think of your 1999 cell phone, if you had one. Think of what you can hold in your hand now.
Any guesses as to what the next 20 years will bring, or the next 300? 3,000? 3,000,000?
Interstellar travel with our current technologies is simply impossible. It's looking like interplanetary travel is much harder than first thought. Even so, what are vast distances to us may not be vast to other species. Traveling the speed of light may be attainable. You can make no credible comments on the nature of interstellar craft, nor its cost.
You can make no credible comments about the communicative abilities of ETs. nor how time would work for them. We don't have a clear idea of how common earth-like planets may be, however, we have only begun to look, and the Milky Way is estimated to have 100-400 billion stars.
You have absolutely no basis to say that all UFO sightings are of inventions made on this planet.
All advances societies have economic systems. Worlds capable of space travel would need very sophisticated economic systems in order to fund their programs. That means that their space programs need to generate an economic return and their probes will have to bring back the goods in order to justify the cost. Are we missing any of our stuff? If not, there are no space aliens on the planet.