There are reasons why some schools have vacancies, not all of them maternity or jury duty.

Just to clarify, in New York City, there is something called the Absentee Teacher Reserve Pool, which is comprised of many teachers who were placed "in excess" for a number of reasons. (For example, shrinking enrollment shrinks the school's budget, meaning there is less need and less money for the same number of teachers.) Every month, teachers in the pool get an assignment to report to a certain school as a substitute. That assignment can be any place in the borough (i.e., the county). Not all parts of Brooklyn are easy to travel to as the subway system generally moves people toward Manhattan. Many of the streets in old neighborhoods are very narrow for the amount of traffic they get today. And don't even think about parking.

Moreover, there are many reasons why schools are low achieving, and many of those reasons have little to do with the students themselves, but none of them work well for a substitute in the midst.