This recent Huffington Post article has a good summary of the issues involved, why they are important, and why they are much more difficult than we might at first think. It describes some of the possible solutions to the Paradox proposed by various scholars. It has also been discussed by Oxford philosopher Nick Bostrom in this 2008 article. However, they missed one explanation:
In 1991 I wrote an article that provides the explanation. Search on "Three Futures for Earth" (with the quotes). It is based on a thermodynamic analysis that leads to several conclusions:
1. There are fundamental limits to what can be technologically achieved, no matter how "advanced" the civilization, and we may already be approaching those limits, except perhaps in a few directions, such as an FTL drive;
2. Given those limits, it would not be rational for any long-enduring civilization to try to live scattered on the surfaces of planets. The only sustainable living environment would be urban biospheres ("starship cities") sited below the surfaces of planets with hot cores, which would provide the little energy they would need, while recycling almost all materials, and emitting only a little waste heat; therefore,
3. There are no Type I, II, or III, civilizations, because no civilization would have any good reason to become one. Anthills don't need to capture and use all possible free energy. They only need enough to sustain themselves as anthills;
4. If an FTL drive, such as an Alcubierre drive, becomes possible, then it would make no sense to waste energy using electromagnetic radiation to communicate. It would be cheaper to travel to places and deliver messages directly. If EM radiation were used, it would make no sense to waste energy broadcasting. The only rational method would be narrow-beam directed at the recipient, with little or no leakage;
5. If FTL is possible, then we could have outposts of a million alien civilizations beneath our feet and never know it, provided they got along with one another. We may not be a "nature preserve", except perhaps to a few alien scientists. To most we would just be barnacles on the ship. If FTL is not possible, we might only have a few dozen, deep enough that no drill will ever reach them;
6. The ultimate end of human development is likely to be single networks of running computer programs (except the computers will be quantum). This is the "singularity" foreseen by some futurists.
It is a mistake to presume that any technological civilization will always expand its size, range, and use of energy to the limits of physics and technology. Human civilizations have tended to expand, but they also tend to fall apart, and not necessarily when resource limits are reached. There are also economic and manageability limits that are subtle and complex, but which can be expected to bring growth to a halt well short of what physics and technology might allow. There may also be a natural tendency to evolve in a way that leaves the civilization with no interest in the kinds of things that would interest us.
It is also a mistake to presume that technological progress will continue forever, along every course. There is nothing inevitable about that. Every technology is an exploitation of a limited opportunity provided by nature that, like an ore deposit, is eventually exhausted. Eventually, one can expect to find no more such opportunities. We don't know where the limits are, but we can be sure they exist at some point. It is likely that most alien civilizations older than ours have long since reached those limits, and that we may not be far behind them.
So if we eventually become aware there are, say, 20 alien civilizations with outposts on this (not "our") planet, we can tentatively conclude no FTL drive is possible.
There is no need for interstellar journeys to be one-hop, single-generation (assuming aliens even have generations). Just put an outpost on some suitable planemo (defined here) (planet not tethered to a star), with a hot core, of which there are likely to be trillions within this galaxy, then wait to drift near another suitable planet or planemo, millions of years later, and hop over to it when close. It would be a spread over the course of millions or billions of years, not less than a few hundred.
Scientists have proposed many alternatives to chemical rockets, from solar sails to Bussard ramjets and more. No need to carry reaction mass or accelerate rapidly. Antimatter drives would work nicely, but are not needed for slow spread across the galaxy.
Aliens don't need to send living beings. Just send a small nanofabricator and the information on how to make them (assuming the aliens even have corporeal forms) if and when it arrives at a suitable location, using materials it finds in the vicinity. Such a starship could be the size of a flea. Just set trillions of them adrift among the stars. Some of them would eventually find and colonize every suitable spot in the galaxy. Such nanofabricators could burrow through many miles of rock, like some seeds or larvae do for shorter distances, and create deep colonies without any of them having any interest in traveling back to the surface or beyond.
If there are deep alien colonies and some of them are sending "away" missions to explore life on the surface, or perhaps launch more starships on million-year journeys, we do need to explain why we don't find openings they could pass through. If reports (video) of the TR-3B (allegedly reverse engineered from recovered alien craft) are true, and we can reduce the effective mass of a large object within a bubble field, then it is not too much of a stretch to imagine a bubble field that would turn a large object into what amounts to a large boson, able to pass through (fermionic) matter without being affected by it. That would enable them to transition from deep colonies to space without passages.
If reported UFOs are carrying protoplasmic beings, those are almost certainly not full "citizens" of their civilizations, but one-time-use synthetic constructs created for the mission and then recycled when the mission is complete. (Think Blade Runner.) "Greys" might be alien, but not "the" aliens in charge, which are more likely to be quantum computers, not biological beings.
Once a civilization reaches the end of possible technological progress, and few threats remain, "curiosity" about further scientific and technological discoveries becomes a waste of precious energy, and any durable civilization is likely to prioritize energy economy and conservation, which would also make them difficult to detect.