Mine is probably the most boring: There are many intelligent species in the universe. Faster-than-light travel, however, really is simply impossible, meaning that there cannot exist a truly interstellar civilization. So while some species have probably settled solar systems other than their own through generation ships, suspended animation, time dilation, or whatever, their range of expansion is limited. This means that encounters between species of different planets are rare. Humans will most likely never contact any intelligent alien species, at most one or two. We might, however, discover evidence of their existence through telescopes or something.
Firstly, that is a very big if. There isn't any proof that FTL isn't possible either and so solving the Fermi Paradox that way is as useful as saying life was created only on Earth by God.
But let's assume that FTL isn't possible. Well, the Fermi Paradox is still not solved. The universe is 14 billion years old. Within a few hundred million years, stars, followed by galaxies, had begun to form. Milky Way is at least 13 billion years old.
Now, life on Earth began pretty quickly after the Earth was formed, which itself was pretty quickly after our Sun was born. All this happened within the last 5 billion years. So life has taken at most 5 billion years to evolve to our present intelligence. Let's say that is the average time needed for a space faring civilization to evolve.
Given that life on Earth started really quickly after our Sun formed, and the earliest stars in the Milky Way were born 13 billion years ago, there should be life that began along with them. Taking 5 billion years to evolve to our present intelligence, that means there should be intelligent life in the Milky Way as early as 8 billion years ago. That life had 8 billion years to travel around the galaxy.
Now, Milky Way is only 2 million light years in diameter. This means our space-faring alien civilization would need to travel 2 million light years in 8 billion years. That is 0.00025 light years in a year. That is a bit more than 2 billion kms. The Parker Solar Probe will travel at 700,000 km per hour, or 6 billion km a year, more than thrice the minimum speed needed for our space-faring civilization to reach us. "Designing something to go fast in space is pretty much the same as you would design it to go slow in space; space has nothing to really impede its progress," was what the project manager for the probe had to say. So there is no reason they wouldn't reach Earth much, much sooner than this. After all, they have literally 8 billion years to progress scientifically.
If life is there in the Milky Way, which it has to be if life is there anywhere else in the Universe, then we should have already interacted with it. Well, more accurately, they should've already interacted with us. And thus, the Fermi Paradox.
This is a wild assumption to make. Fuck's sake, we're not a space faring civilization. Let's say space is an ocean - we're a civilization that has gone up to the shore and walked around in the tide - arguably we've done a bit of swimming but we're still right up against the shore, and has launched a few messages in a bottle in the hopes that maybe someone will find them. We're not a space faring civilization.
Yeah, for sure, when you're thinking about it in terms of a human lifespan. But the time scale we're talking about is insanely huge. So your argument doesn't make much sense.
The current estimate for when life on earth began is actually 3.5 billion years. But I was being generous with the numbers. In this timeline, we have 1.5 billion years left to become space-faring. You gotta remember the scale, man.