3. Why are military leaders concerned that only a small number of counties reliably produce soldiers?

This is because it is not unsustainable and also unfair in a democratic society.

Explanation:

For decades now, most of US soldiers have some from a certain demographic. The age bracket, education and even race and even geography plays a big role.

For example, the North-East of the United States is the most densely population region in the country but produces far less recruits than the Southern parts of the country.

This not only creates a civilian-military divided, but can restrict the talent and skills the army is constantly recruiting for. The more diverse the pool of recruits, the better equipped the US army would be.