But still, it's just unfathomable that in billions
of years across billions of trillions of star systems,
that only one developed intelligent life
Only a vanishingly small portion of them will develop advanced tools, much less explore the universe. We have some pretty intelligent non-human life here on earth, but it doesn't seem like squids, elephants, or dolphins are on an upward intelligence trajectory and seem unlikely to explore the galaxy (apologies to Douglas Adams w.r.t. dolphines)
It's also important IMO to understand that advanced intelligence is not an inevitable goal of evolution. Evolution only "cares" about survival. That can result in intelligent life, but it can just as often result in horseshoe crabs -- creatures of low intelligence that fill a niche perfectly and resiliently.
Intelligence is a really tough way to survive. You need a metabolically expensiv and fragile brain (or brains)
I'm a believer of the notion of "great filter(s)."
I think most civilizations on a path to interstellar travel have to pass through a nuclear fission phase (how do you build advanced stuff without discovering this?) and I think a great % of civilizations will end/cripple themselves during this phase via nuclear war. This will filter out a lot of civilizations.
The next challenging filter would be making serious efforts at exploring/colonizing/exploiting other planets and solar systems before exhausting their own home planet's resources or making it unlivable. The human race is definitely on pace to lose this race.