The core trope is based on equating wealth and education with native intelligence - it's not smarter people who reproduce less, it's affluent people.
The people aren't so much stupid as they are incompetent, which is distinct from stupid but not as easy to explain to the audience (irony?)
And while its not 1:1 the correlation does actually exist. inheritance, nepotism and failing upward aside, as a general rule intelligence promotes competence, and compitence gets you better jobs. Being book smart but incompetent gets you nowhere.