Just took a snapshot of worldwide flight activity about 15 minutes ago. Interesting. Bear in mind, these all aren't passenger aircraft.
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These two videos might explain the amount of "traffic" in the air.
What's the TL;DR? Both of those videos are 15-20 minutes long.
When they're not flying, they're parked, even when they're parked they have to be maintained. Sometimes flying them is cheaper than parking them, or they don't have a place to park them.