the most likely explanation is so simple...
the Universe is huge, and in order to get to a civilisation like ours a shitload of random things have to go right over the cause of millions of years.
and even tho this all went right for us, even we can't roam the universe yet.
the fact that we even evolved into a species that can think the way we do is extremely random and probably super rare.
there's no unique evolutionary benefit for our intelligence, it just happened to go down that route and we managed to survive, yet we are the only species on our planet to have this capability, with our closest related animals maybe reaching the mental capabilities of a human toddler.
intelligence is not necessary to be evolutionarily successful. the most successful species on our planet are among the dumbest organisms possible. they are barely sentient... your phone's AI assistant is probably more sentient than most insects.
they just run a simple routine, no individual thoughts in sight, like preprogrammed machines.
so everything that has to go "right" for advanced civilisations to exist is far more insane than many seem to take into account.
and not only that! it also has to happen close enough to us for them to even realistically be noticeable, AAAND they have to exist at the same time as us, and be already sophisticated enough to make themselves noticeable for us.
so a shitload of random things have to go right to get to our level.
it has to go right close enough to us to notice.
and it has to happen at the same time as our existence lasts.
that's like winning 100 lotteries in a row...