reading the negative reviews reveals the film's true theme
It seems many viewers were so distracted by the style of the film that they missed its substance. To me, the film is not a critique of suburbia or parenting but a horror film about the positive qualities of both. Spoilers are full steam ahead below...
Humans are excellent parents. We parent our helpless infants for a shocking long time, we build safe houses for them - rain, lightning & earthquake-proof. We raise them in patterns; sweet quiet streets & cookie-cutter homes, cookie-cutter gender roles as parents. With such bountiful generosity & devotion, with such easily grasped roles, how could a brood parasite not take advantage?
The aliens in this film are our cuckoos. They give us their child and we raise it in our own nest. We are trapped by our own humanity. When we see the newborn we feel parenthood instinctively. We cannot crush the newborn's head with a pickaxe, though we know it is not human. We feed & nurture the child - perhaps begrudgingly, abusively, neglectfully - but we feed and nurture it even as it grows larger than ourselves. It takes all our energy to care for it, draining our very lifeforce, but we keep doing it. We can find no other option... We are trapped by our own nature.
Other critics seem to think the aliens lack motivation, or that they have totally alien motivation; researcher motivation. I don't think so. Like the cuckoo, they are only fulfilling their own nature. They cannot save their para-parents. They are doing this to survive - it is their role in the circle of life. Why? Perhaps because their life span is so short, perhaps because their extra-dimensional nature makes typical parenting impossible, perhaps just the quirks of evolution. They look at us as we would see a cow. And they know how valuable our milk is. They must drink it to survive, so the baby calf is turned to veal and we eat ice cream on hot days. But are they evil? Not unless your tapeworm is evil. Nature is amoral.
It is our own morality - the love for a baby's face - that traps us. We cannot dig out of our morality, or outrun it, or outthink it. This movie is exceptional because it shows us how our freely given empathy could be used to entrap and enslave us. I guess the only escape from this trap is infanticide or suicide - making the brood parasite strategy untenable, but ending our own life in the process. A more cruel trap than Jigsaw ever made. Horror at its finest.