Why are plane tickets so expensive?

Related questions

1. Do flight prices drop day of?
2. Are flights really cheaper on Tuesdays?
3. How far in advance should I book a flight for best price?
4. Do flight prices go up the more you search?
5. Is it cheaper to buy tickets at airport?
6. Do plane tickets get cheaper last minute?
7. Will flight tickets go down?
8. Is it cheaper to book flights after midnight?

Tags

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14. #things
15. #airline
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17. #expensive
18. #gallon