Because the black goo contains genetic code from Engineers, and humans were originally created from the genetic code of of Engineers. The appearance off the Offspring suggests that there was a genetic throwback of the portion of the Engineer's genetic code.

Because the Engineers created humanity from their own seeded DNA, the Offspring showcases those deep underlying traits. That’s how I took it.

Strongly dislike the concept that we relied on one singular humanoid species to create us humans then try to kill humanity for no reason, absolutely blasphemous