Could also be some sort of spore-spreading aliens. What matters is that they reach some end-stage that squirts spore/asteroids into space to reach other planets, and the actual consciousness/intelligence of the individual aliens is in fact a byproduct of how they spread, like in Selfish Gene theories.
However, all of that was in fact irrelevant to the type of story the author wanted to write.
https://www.reddit.com/r/manga/comments/3l5p58/disc_why_did_parasyte_end_the_way_it_did/rather than typical alien vs. humans, Parasyte focuses more on the way humans see parasytes and the way parasytes see humans and ultimately questions what does it mean to be human.
The fact where the aliens came from didn't matter at all.
No more, and no less than that. Maybe the story could have gone on to be an adventure story where the "true" aliens invade and then there's an arc about that, with all the politics and international drama, but that wasn't the type of story the author set out to write, that wasn't in the "scope" and doing so would have watered-down the concept. Sure, you can add those things and the reader/viewer is free to imagine the story going off into any possible direction, but building that into it would have just eroded what the author in fact set out to do entirely.
I think the core story in Parasyte is about the aliens pretending to be human, and how they understand it in varying ways and to varying levels, as well as the conversation/conflict between the alien way of thinking and the human way of thinking. For that to work best, the aliens knowing nothing of their "origins" actually works better since the aliens are trying to understand human-ness while lacking any point of reference of their own. Going too far off that would have watered-down the concept. The need for action / events already watered that down quite a bit, but you need that to keep people reading it. Similarly, the reason it doesn't go international and look at how the parastyes infect all the nations is that doing so would have eroded it away from being a
personal story about the MC and Migi interacting with the other parasytes around them, and how they blend into human society.
I remember Neonivek was good at those things, asking "why didn't XYZ happen in this series, it would have been more realistic", when in fact "XYZ" was completely out of scope of the genre of said work, which someone like Neonivek who thinks a lot about fiction should have been genre-savvy enough to realize. For example, why can't one of the girls in K-On be hit by a car and spend the rest of the season in a coma, with the other girls all visiting the hospital and being all sad and shit? Well, it would be "realistic" because things like that do happen. But it would be completely outside the scope of the "school slice-of-life comedy" genre, so it
just can't happen. Genres matter, and strongly constrain the scope of where a story is allowed to go. A slice-of-life comedy story can't have such a "crisis" happen, because if it could then it would retroactively be a story about said crisis, and not a comedy at all. Similarly, if you have an action series, and the characters decide to settle down and from that point on, it's a slice of life drama story about domestic living, viewers would
flip their shit despite it being perfectly "realistic" that people do decide to just chill for a while.