Probably get the numbers a bit wrong but there wasn't much time before the evolution of life on earth to evolve intelligent life.
life is made of chemical elements. Those chemical elements are formed in stars. A first generation star made primarily of hydrogen has to form, burn, produce the chemical elements and explode. Those elements then need to coalesce as a second generation star surrounded by rocky planets before the environment conducive to life even exists. So how long does that take?
Earth is 4.5 billion years old. The universe 14 billions years old. It took 10 billions years to create the earth from scratch.
and how long does it take to form intelligent technologically advanced life once the environment is set up.
2-2.5 billions years for bacteria to evolve.
another 1.5 -2bn years for complex animals.
another half a billions years for intelligent animals with advanced technology.
there wasn't really therefore much time to complete the evolution of the appropriate environment and the subsequent evolution of technologically advanced intelligent life.
furthermore, the evolution of complex life on this planet involved numerous highly improbable chance events, from collisions with other planets creating tilt, seasons and a moon which generates tides. Just the right amount of planetry mass, an atmosphere and tectonic plates. Being Just the right distance from the just the right sized star. The evolution of single cellular self-replicating bacteria is a miracle in itself involving the evolution of complex metabolism, a genetic code, and the biological machinary to translate code into proteins into life. If thats not amazing enough, then the evolution of complex cells required the an association of different types of bacteria coming together and existing as a single life form. The evolution of viruses working in conjunction with bacteria is essential. (Mrs carl sagan knows quite a bit about this).
Complex single cells need to evolve into multi-cellular life forms which then have the opportunity to specialise different cells for different functions, and really complex animals can begin to emerge. Water is probably essential. So is lots of climate change, volcanic activity, and possibly a few impacts from asteroids which forces life to undergo various transformations to emerge onto land, generate hands, eyes, and large brains, language, culture, tools and ultimately perhaps weapons and advanced technology. Maybe the animals that achieve all this are inevitably top predators - aggressive, war mongering, and necessarily selfish.
It is perhaps worth remembering that 99% of the way through this process on Earth the dominant intelligent predators were reptiles. It takes numerous chance events such as asteroid impacts and volcanic eruptions to continually clear the decks and pave the way for the evolution of animals with sufficient potential to be technologically advantaged. Large brains required the dominance of animals with long gestation periods and live young, which in turn required periods of dry climate, trees, climbers forced to start walking upright and collaborating etc etc etc etc.
the reason why there is a real doubt about the possibility of advanced alien life is because to achieve all of this required a huge number of seemingly chance events. Its like rolling a 6 maybe a thousand times in succession. And even if it happened twice, the window of opportunity for two technoogically advanced intelligent animals to exist at the same time is minute. How long has intelligent technologically advanced life - with the potential to launch itself into space - existed on this planet. 50years in 4.5 billion years of Earths history. And how long will that animal exist before it destroys itself. (We are already responsible for the greatest mass extinction event in the history of the planet).
And finally even if two technologically advanced life forms ever existed at the same time, during the same tiny window of opportunity, the chances are that they are too far apart to ever meet.
We are alone.
better start looking after this miraculous planet.