>There's a vast gulf between "life exists on multiple planets in the galaxy" and "that life is flying around Earth in little metal spaceships that defy all known physical laws".
The universe is 13 billion years old. The Milky Way is about 100K lightyears across. Suppose another species could travel at 90% of light speed. Then we're looking at maybe 110,000 years necessary to colonize the galaxy -- only a tiny fraction of 13 billion years.
I've been following this UAP stuff for a while, and as far as I can tell no one is claiming that the UAPs defy all known physical laws (i.e. I've seen no claims of violation of conservation of energy, 2nd law of thermodynamics, FTL travel, etc. -- though, someone should do the math on those transit times to check the FTL travel one). Just that the propulsion & cloaking work in ways we can't explain. Show Aristotle a jet fighter and he'll have a similar reaction.
Supposing that the UAPs arise from a civ that arose when the universe was about 12 billion years old, i.e. 1 billion years ago. That's about 1 billion years of technological development. Can we really put confident bounds on what could be achieved in terms of propulsion and cloaking with 1 billion years of research?