The Fermi paradox:
If only one species masters interstellar flight (NOT
FTL, but BELOW LIGHTSPEED flight), then this species will easily colonise the entire galaxy within 100 million years. Also - after it spreads to ~10 solar systems, this species is all but indestructible - no catastrophe can extinguish it any longer.
Well, our galaxy is ~13,6 billion years old - but only since ~6 BILLION years there were enough heavy elements for life to be able to form.
In ~6 BILLION years, if ET advocates are to be believed, many, many intelligent species have arisen. But these advocates always have trouble explaining why none of them managed to send the first few interstellar ships - a feat humanity is close to accomplish (a few hundred/thousand years is a mere blink of cosmic time).
EVERY SINGLE ONE of these species - all the factions with different agendas and actions each species was composed of - always conveniently went extinct or 'insert another solution to the Fermi paradox that badly breaks Occam's razor'.
The most likely solution to the Fermi paradox is either that abiogenesis is very hard/rare (which is heavily supported by the staggering complexity of the simplest molecule that can reliably self-replicate) or that technological intelligence is very hard to evolve.