Because its premise is that stupid parents exclusively get stupid children, and stupid parents gets more children than intelligent parents. That is how the world ends up as it is in the movie.

But that is of course not how intelligence works. Stupid parents can get intelligent children, otherwise intelligent people would never have existed in the first place.

Of course that doesn't make the movie embrace eugenics. It is just an unimportantfictional premise meant to explain why the world is at it is in the movie.