eli5: How are airline companies profit margins so thin?

Question

Theres like 5 companies in the US that handle ALL the flights. How do they still have thin profit margins despite being the only options?

Edit: spelling

In: 72

Answer 1

Competition. Many highly demanded routes have at least two, and sometimes more, airlines serving them. This prevents a single airline from trying to charge excessive fares as their competition could be flexible enough to undercut them.