One big error in the fermi equation (which started the fermi paradox), is that it has completely discounted the possibility that a civilization could cease to exist once it has reached type 3 statue (a type 3 civilization is one which has colonized multiple star systems.) One thing that could destroy a civilization is internal stagnation, After a few thousand star systems had been colonized, or even less, it would have been virtually unavoidable for all relevant technological advancements to have been made already, and as such there would be no more of a use for expansion. Such is what happened to china after two or three centuries of innovation. If two hundred years could do that to a civilization, imagine what two hundred millenia could do. After a half million years, all that could be left of a once powerful type 3 civilization could be a few thousand bored immortals and a few million terraformed worlds left over from the golden age. Another thing that could destroy civilization would simply be bad ideas. Any intelligent society is capable of being destroyed by ideas, and the more advanced the society gets, the more advanced the ideas get. Many convincing ideas would no doubt shatter even the greatest galactic empires. As such, if there were 2,000,000 type 3 civilizations in the milky way, and each colonized 10,000 star systems before declining, there would still be only 20,000,000,000 out of 200,000,000,000 star systems in the milky way colonized, or 10% of the star systems in the galaxy. As such, it is rather unlikely that humanity is currently relevant to the lives of any of the major galactic powers.
We are in a very short time period in our own solar system lifetime. Our sun alone took about 10 billion years to evolve into it's present state and humans have only been around for about 200 thousand years. Other planets that could sustain life may still be in the infant stage. Some may have already have gone extinct because their sun gave out. Humans also caught a break when the dinosaurs died out. Had that not happened humans would not exist, and it is doubtful that the dinosaurs would have evolved as humans have. So even though a planet may be of the same age or older, it may have life that was unable to grasp science as we have.
Space: As advanced as humans are, the farthest we have sent people is to our own moon and back. We only did it for few years back in the late 60's early 70's and have not gone that far since. The fastest we traveled in space was about 24,790 mph. I think the closest earth like planet is about 20 light years away. That's about 117 trillion miles. At that speed, it would take us about 539 years to get there.
Basically, IF even the closest planet to us had life, it may not have developed the science of space travel and even IF they did, and IF they knew about us or about our planet. Do you think they would bother sending a crew that distance? IF they did, maybe it is still on it's way.
Say a planet has life. The closest star is 4 light years away. This planet is, say, 500 LYA, which is still very close. It sends out a radio signal. Now, this radio signal lasts for about 60 seconds. It has almost astronomical chances of being read by humans. If the signal was sent out in 1100, it would reach here by 1600, when we didn't have the technology to pick it up. If it was sent in 1510, perhaps it was sent on a frequency not being recorded at the time or not readable by our machines. We have only had the technology to be able to pick up these signals for 100 years. The age of the universe is roughly 12 billion years. So this contact or evidence would've had to happen in the EXACT right time, which is very, very close to impossible. Plus, we've had the WOW signal.
The implausibility of interstellar travel is the best explanation in my view. There are sociological explanations (e.G. The prime directive, lack of interest, etc.) which are not so convincing. There are other explanations, such as galaxy urbanization (i.E. We're in a "rural" part of the galaxy), or that civilizations do colonize the galaxy periodically but then die off.