Purely conjecture and "running with the ball" here for the sake of discussion. I've no dog in this fight.
Why the assumption they come from outer space?
If you want to believe something they are just as likely inter dimensional beings or time travellers coming back which would certainly explain their timidity at being seen up close and personal interaction (time paradox issues).
It would also help explain the fairly twee ecological lectures/images they plant in the minds of people (alledgedly). Maybe the're trying to warn of us an upcoming ecological catastrophe that made future humans end up looking like them.
Maybe they're on a time travel package tour?
Some theorists posit they are coming back through time to harvest genetic material from us that they are now lacking due to some future catastrophe.
It would also explain why the majority of sightings describe humanoid figures quite similar to us. An space type alien is just as likely to be microbial or giant in size comapred to us.
Many sightings are of USO's (unidentified submerged objects). Who's to say they don't live down there in relative safety from prying eyes. The oceans are quiet. It's specific shipping lanes that are busy not the rest of it. Just like that film "The abyss"
There's no evdience linking them to space travel except what the so called aliens have communicated to contactees. They always tend to say they're from Zeta reticulii but it may as well be Bognor IMO. Contactees are often in what I'll term a fugue state or altered state of conciousness. The "Oz" factor as described by Jenny Randles. Another researcher I'd recommend you check out.
Like I said it's pure conjecture but the logic they come from outer space is no more likely than these other theories basically. It's a can of worms. A can of weird worms.
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Apologies for the double post and for the poor spelling in my prior post. I'm trying to do too many things at once.
I'm including another link which you could counter with in the interests of balance but it's been kind of increasingly discredited over the years. It also explains my Zeta Reticulii mention prior:-
en.wikipedia.org