There is no conclusive evidence that there even exists such a thing as interplanetary travel for a life-form. We've barely touched the moon ourselves.
Now, granted, the acceleration from the beginning of the last century to the Moon-missions was extraordinary. But since then, if anything our acceleration has slowed to an absolute crawl. The expense of a simple one-off mission that we've already done several times just isn't viable any more.
Now, consider, that you could get to Mars. It'd take decades of planning, travel, etc, but you could get there. That's the nearest planet.
Now don't consider distance, etc. necessarily. Consider resources. Now you have to find the time, money, resources, engineering, etc. in order to make fuel to make the next jump. That's not easy at all. Hell, Mars is being talked of as one-way at the moment. And if we got to there, to get to Jupiter would take even more resources, energy, etc. Now there are ways and means to cheat this, but they are slow, and not capable of sustaining human life along the way at the moment.
But let's say, on every planet we visit, we find a ready-built space-base with fuel and oxygen enough to get to the next planet. We land, breed like fuck, and it only takes 20 years - doing nothing else - to plan, fuel, and travel on to the next. That's nearly two centuries before you're heading out of solar system. And you're unlikely to be overtaken at any point, even if Earth finds an energy source 10 times more powerful in that time.
Asteroids - even less resources, even harder to land on, even more difficult to colonise. Let's say we fire out probes all the time we're doing this (ignore where the resources for these probes comes from).
The next star is 8 light years away. Let's assume every star is that far away from the next, every star has the same kind of planetary system, etc. It's going to take several centuries to get to the first. Several millennia to traverse a handful. Meanwhile, all the probes your sending out will barely hit the next star but let's say they hit 10 stars on the way out, and talk back instantly if they find something. We could cover a few hundreds of stars in that time.
Let's go mad... several millennia of this (we'll stick with c as the limit of physics, but that might obviously change - at that point, we'll reconsider Fermi's Paradox anyway!), and the entire race dedicated to populating a planet, building the infrastructure to convert every resource it has to nothing more than space travel "fuel" (of whatever kind), and their sons move on to the next planet, all the while sending out hundreds of probes. Every few centuries, they go to a new star.
That's, rounding UP, (10^4 years / 2 x 10^1) generations, 10^1 stars per millienia in each direction. The orders of magnitude wouldn't get near 10^8 at all.
Do you realise where that gets you? There are a hundred billion stars just in our galaxy. That's 10^11. It'd take thousands of millennia (millions of years) to do this at stupendous speed across the galaxy, stopping to do nothing else.
No doubt there'd be advances and speed-up, but you're still orders of magnitude in debt before you've colonised a galaxy sufficiently. And then you consider the number of galaxies - That's another 10^11 or thereabouts.
And then you add in real-life, where we aren't just able to do nothing but look for aliens. What you're suggesting is that, even if there was a civilisation just a few stars away from us (incredibly unlikely given what we can see), it'll take anywhere from centuries to millennia to discover them. Assuming speed-of-light all the way, and communicating with probes all the way, etc. it'll take longer than man has so far existed in a form capable of doing such things to actually make any kind of contact if only, say, 1% of the galaxy is habitable.
The numbers just get more ridiculous after that.
Now, of course, we're limited by our current knowledge. But that's the point. Our current knowledge says there's nothing even in range. And even with exponential increases in detection, capability, resources, dedication, etc. we're still talking millions of years. And until we can enhance that knowledge, even assuming we can do the impossible of a "Moore's Law" for such things, we're still not in the right ballpark to actually find anything with any certainty.
And it's more likely, in those millions of years, that we die out entirely, especially if we spread ourselves so thin. Fuck, we're not sure we'll make it off the planet ourselves - we have literally never sent a human being to another planet, ever. If we're lucky we might get a self-sustaining colony going somewhere before the Earth kicks us out. But even then, something on Mars, or Alpha Centauri, is NOT as hospitable to life as Earth currently is. Sustaining a population of any significance would be nigh-on impossible. But we just assumed that every generation, we could just build a new shuttle, fuel it, launch it, etc. just using the resources of the planet we've landed on and leave mum and dad to rot on the planet we've just stripped bare and that was none too hospitable when we arrived.
The problem is not lack of imagination, it's just a sheer numbers game. Until those numbers change significantly (they change all the time, we've revised all sorts since we started detecting more planets around foreign stars), we're up shit creek without a paddle. Even if there are a thousand other civilisations doing exactly the above, all looking for each other, it's going to take extraordinary feats of science, and hundreds of millennia, for them to even get close to detecting each other. And then when you run the maths for the chances of them crossing over at a point where they recognise and detect each other, it actually gets even worse.
Personally, I believe in the unofficial science line here - sure, there are likely civilisations out there. They may be more advanced, they may have technology orders of magnitude greater than ours (and thus also orders of magnitude less impact than we might be looking for, if they are at all conscious they can be seen), they may dedicate their entire civilisation to finding us, for millions of years, AND STILL NOT FIND US.
That's the problem. It's not naivety to make up fantabulous numbers like this, run them, exaggerate every possibility, and still find that it's incredibly unlikely that anything significant would ever happen. It's not naivety to extrapolate that back to more realistic numbers and say the same.
The problem of space is distance and time, and a speed limit. Even breaking the speed limit, and making the time as short as possible, and taking best cases for distance, and exponential advances in technology? Still unlikely that what we see in the galaxy will change significantly.
And, unfortunately, all we see in our galaxy are inhospitable places, vast distances, and not enough time - in our civilisation or even our universe - to make a dent in them.
