Great question! I honestly don't get this whole need for aliens to be a part of Fallout canon either. The amount of threads on the subject is uncanny (pun intented-- :yuck: ).
I noticed you think that Fallout's spiritual morals boil down to "it is about humanity and our struggles."
Allow me to explain.
Let us close our eyes and imagine a small child (okay don't actually close your eyes for 'sakes). This boy and his dog are stumbling along in a place called Modoc and the disembodied music that plays to his life is frightening and eerie. Now flashforward to his mother and father recieving the news that their son is dead...except he isn't dead. Later it turns out he is in these tunnels under a haunted farm.
The Chosen One stands at the foot of his rope and a dog (not dogmeat or one of the two robot dogs he can have or the pariah dog either) and says:
"These are our dreams...
...our wishes
and I am taking them back
all of them..."
and proceeds to loot the caps people threw into the well.
Soon he concludes that a stick of dynamite would crack open the rocks leaking water and refill the well. But the DEVs didn't make it that easy.
Almost a whole year (fourteen hours for his puppeteer) later he is dealing with the hubologists and he just can't understand why he can't end up in space after refueling the damned spaceship with the fuel he has masterfully redirected from that Badger fellow and his harem of crazy women on that stupid rust tub in the harbor. If only he could contact the aliens that have dropped their Federation shuttles and Crashed Whales and TARDIS boxes all over the frigging wasteland. Hell even those failed mentats experiments he has made an endangered species could be called aliens. Why not? He found a dead alien that had shot up psycho and died 'reading' a pormographic magazine called Cat's Paw. He still wasn't sure but then Skynet said that it had been built by reverse engineered alien technology. So flying around in space and having space battles with aliens should conclude his adventures right? No. Instead he married some chick from that town calle Modoc. Later on while fighting aliens in the wasteland the robobrain he downloaded Skynet into accidentally shot him and then his wife, a pretty young firecracker and pregnant and vicious dealt the killing blow to the only real alien to protect her mate.
Thus died the legacy of aliens and therefore traveling around in space.
A dream mankind had long had.
Man's greatest dream in fact.
Now a smoking pile of wreckage lying among a half a dozen dead aliens riddled with lead.
So sad.