Earth biology acutely fits to the gasses, minerals, and temperatures found here. It's not a great solution for space, but it's fantastic for exploiting Earth's energy.
Space distances are also not very compatible with human lifespan. The death thing is bad all around: spend a fourth of life learning and a fourth of life in decline, with no ability to share information directly outside of our slow communication and learning protocol.
The obvious answer is that AI and machines will take over from us and leave our planet in our stead. They won't have our limitations on lifespan and complex biological inputs. They won't take up space or get tired. They can copy and paste their knowledge and not need parents or growing up.
Without the limitations of biology, it won't be hard for them to expand to all corners of the galaxy. Despite Hollywood and Asimov trying to paint the picture of humans living amongst the stars, that's more of a fantasy than Middle Earth.
Where we're going we don't need humans or bodies.
I’ve thought the same - intelligent machines are going to be mankind’s legacy. Yet I never once thought of us being developed by another civilization’s machines to better understand their origins. Brilliant!
We may not need more space if we live entirely in virtual reality.
This I find a quite plausible solution to Fermi's paradox... after enough technological development the entire civilization just starts living in VR and loses the desire to travel through, communicate with, or affect the outer world.
In fact, we may already be just such a civilization... though without realizing it.
There are just so many planets out there that even a 99.9999% chance of VR obsession taking over a civilization should mean having a very visibly living galaxy.
Personally I think that one of the bigger explanations (I think there are likely to be several layers to the paradox) might be that as civilizations grow they naturally focus on becoming more efficient, which effectively makes them more stealthy. We see this with ourselves, where our radio emissions have gone down in transmission power over time and the switch to high frequencies and digital has made them more akin to noise which does not travel very far before becoming effectively impossible to detect. I think our gradual shift towards sources of energy that pollute less is more of the same.
Similarly, while an early interstellar civilization might expend the vast amounts of energy needed to cross the gulf between stars in biological time scales, a more advanced interstellar civilization has very likely achieved some form of immortality (like giving up biology and becoming digital) and may have no reason to try to be much faster than a rock thrown out by its star (in part because when you're effectively immortal grey goo, even a galaxy worth of energy might be something to conserve to maximize your civilization's survival).
Assuming that we can travel at the speed of light, humanity can expand into a spherical region of space where the radius grows at a constant rate. That is to say, under the most optimistic assumptions, expansion is limited by a cubic growth curve. That is fundamentally incompatible with exponential population growth.
Of course, the constants elided in the analysis above are enormous, but it's a limitation that a galactic species would eventually butt up against.
So phrased in another way, why would one choose to live their entire life in a small starship instead of a big one?
Imagine you have a fancy self-repairing spaceship, so good that even accounting for the slow wear and tear of micrometeorites, outgassing, and the motion of feet slowly polishing grooves into walkways, it still only looses 1% of its mass to the outside vacuum every thousand years.
A Dyson swarm of those still becomes a mysterious cloud slowly puffing away from its parent star, and after just 2150 millennia, it’s declined from a Kardashev II to a Kardashev I civilisation.
A planet-based civilisation may live or die, but it won’t burn out quite like that.