F-sub-p is going down all the time, isn't it? If you're interested in this sort of thing, you've a. already read articles like this one and b. you're not that surprised. But then Fermi's question - "Where is everybody?" - becomes more insistent.
1. They're there, but we don't know what to listen or look for. This is to my mind overwhelmingly the most likely possibility. Sub-possibilities:
1a. Long-distance electromagnetic communication is a temporary local optimum. We're already moving away from it here on Earth. Unless we catch aliens in the middle of their 1920s to 1990s period, we'll miss them.
1b. Once intelligence is achieved, replicators profoundly altering themselves is not far behind. Are you sure you would recognize even your own "descendants" five thousand years from now? Why assume we'll find a species at a similar "level" of development, even assuming such a term can have any meaning outside humans? (Referring to 1a above, it's a little naive to assume the aliens will have 1950s. Don't rely too much on Star Trek episodes of yellow-blooded humanoids which carry the assumption of nearly identical biological and cultural development.)
2. They're there, but we're not seeing them, because they're intentionally hiding. Once introduced into the galactic ecosystem, organisms either try to conceal themselves and effectively disappear, or disappear for real. Unless you happen to catch a newborn intelligence's careless birth cries, you won't hear anything.
3. Maybe replicator chemistry could be common, but intelligence is not. The kind of replicator chemistry that produces representational tissues (experience-generating nervous systems) and therefore tools that allow them to communicate or move across interstellar space might be a fluke. Sharks have been dumbasses for 400 million years and my money is on their not inventing algebra in the next 400 million years. Intelligence is only useful insofar as it helps things reproduce. Or, if intelligence isn't a fluke, a dead end, one solution Fermi did in fact consider. Life on Earth has produced intelligence once, and it's not clear that those organisms which haven't achieved it have any tendency to achieve it.
4. We've only been looking in earnest for a half century. Asking "where are they" is probably like me looking out my window in San Diego in 2010 demanding "if the Earth has glacial cycles, then why don't I see any glaciers in the canyon next to my house? There must not be any glaciers." (Similar idea here.)
1. We should absolutely not advertise our presence.
2. If designed self-replicating machines are possible, then we should look for those. I predict we will find some in our own asteroid belt and/or Oort cloud. (Shorter version here.)
3. We should be building a fleet of small interstellar probes to explore those exoplanets right now.