Why do I hear airplanes' engine sounds in the sky?
20 km is not that far for an airliner. At the speeds they move over ground, even in approach phase where they move slower than in cruise, they are few minutes from landing or after taking off.

Depending on prevailing winds, the airport might be using an approach path or another. That means some days you'll see airliners overflying you non-stop, while other days you might not see a single aircraft.