It's not just about knowingly purchasing a home under a flight path, it's also about the commitments made by airports and airlines. For example, an excessive noise event may be caused by a pilot not following a proper noise abatement procedure. And while airplanes have gotten quieter over the last 30 years, delivery companies frequently purchase older aircraft that may not be as quiet. As those freight operations scale up, they can create more noise than anticipated. Finally, the use of GPS assisted takeoff and landings can create issues - previously aircraft were more spread out in their takeoff and landing patterns, which would mean more people are exposed to (relatively) less noise. With the increased accuracy of GPS navigation, flight patterns and much narrower, so the concentration of aircraft on a flight pattern means fewer people are experiencing more noise.