Another possibility - civilisations decline and fall, technologies are lost and rediscovered. We are still not entirely sure how the Egyptians built the Pyramids with the technology available at the time, or how the ancient Britons moved the blocks of Stonehenge 200 miles from the quarry science tells us they came from. I'm not saying aliens did it, I'm just pointing out our own civilisation lost that particular technology and arguably never rediscovered it.
It's already been established that even at light speed, it would take a substantial amount of time to get here from any but the closest star systems, so any alien civilisations reaching us realistically travelling significantly slower than light and a far greater distance would have been travelling for generations. Maybe hundreds of generations, maybe thousands.
Perhaps, like pyramids, their civilisation was once advanced enough to design and build faster than light starships and send them on their way, but the civilisation collapsed at some point during the journey. If they remember where they came from at all maybe it is just as a myth or a legend, with their data records corrupted and any written records being recorded in a 1000 year old language no-one understands any more.
Maybe you would have a fleet of starships arriving each one having evolved independently, so you end up having primitive aliens warring with each other, unaware they come from the same place, with the more advanced Earthlings watching on in bemusement?
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