This site has discussed the Fermi Paradox before
Variation on transcendence, aliens advance to some other level.
Something really sucks about our universe or interstellar area. This is somehow apparent to all advanced aliens.
It is likely that traveling around interstellar and intergalactic space is very expensive energy wise.
If it turns out that physics allows certain things to be far more inexpensive energy wise or to provide far better returns for the effrot then it may be obvious to any aliens that it is a waste of effort to travel around this universe or galaxy.
Customizing Universes or Selecting Better Places with Wormhole Travel
Customized Pocket Universes (tough to meet up with other people in the TARDIS) -
If along the way to working around conventional physics to make FTL you have develop ways to manipulate spacetime then it may be by default you have to develop the ability to make customized universes. The cost benefit of traveling around this universe may be low or negative. I can spend the same amount of initial energy to open up for FTL and make a whole other universe and extract energy/build from the exoverse/multiverse. Super advanced aliens then do not live in wild universes but move to customized universes or universes are thus far more sparsely populated It is not even necessary to create custom universes, if multiverse exists and multiverse travel is possible then there could be better universes to find and move to.
It could also mean that it would be likely that most universes are customized but ours could be an earlier one. We could be living in the DOS 2.0 version of universes and everyone else moves on to better versions. We are in multiverse equivalent of armpit, USA and people move to the better universes when they get the chance.
Dark Matter Rockets
Blackhole starships - discusses ways and energy to make small blackholes
Mach effect could be used for fast travel (not FTL in that mode) and could enable wormhole creation and travel.
Wormhole creation and travel could allow multi-dimensional movement.
Before aliens leave they make some super telescopes and look around in detail and see confirm that this universe is inferior and swap out.
Sitting beside a timebomb or Love Canal
Another possibility. Aliens can detect the conditions of stars etc... They see that some stars in our area are going to supernova in the next few millenia. The stars will blow up and make things inhospitable. They decide it is a bad idea to hang around or to invest in building and doing stuff in our area.
Again they all go elsewhere.
Some other Options
There is also a variation on the "god must be crazy" situation. The movie has a tribe that gathers a coke bottle and uses it for tools.
If we are clueless then we could be using alien artifacts and evidence but not recognize them as such.
There is the situation where if you get the tech you do not hang around stars. you have energy better than fusion.
Gathering around natural "campfires" just makes you a target for galactic predators.
So advanced aliens are hanging around the Oort comet clouds or in intergalactic "voids".
Advanced aliens may have to go fully ninja. Meeting up with other aliens is not worth the risk. Many science fiction story examples of this.