One rationalisation is that hiring someone "too smart" for the work required means that they'll soon become bored and move on, so your effort invested in training them is wasted.

For a modern American police force in the Bay Area, for example, taking a candidate from academy entrant to fully qualified officer is a six-figure cost.

That's why many Bay Area LEOs focus their recruiting on recent graduates from other Bay Area forces.

This is exactly what I’ve heard the justification is.