They put airports where there is room to put them. Frequently that means that they are well outside cities since finding space for a few 2 mile runways in the c

They put airports where there is room to put them. Frequently that means that they are well outside cities since finding space for a few 2 mile runways in the city is difficult. However, there are plenty of airports in major cities and built up areas (in both developed and developing countries), and I have never heard of avoiding building in areas due to the (remote) possibility of crashes.