Why is it often considered that firms In oligopoly will collude over prices and form cartels?

It is often assumed that companies in an oligopoly will collude and fix prices because this will maximize each company's profits.

• Let's use the example of the airline industry. Technically, if a plane is about to take off and there are empty seats, the airlines should sell these seats even for $1 since they would otherwise make $0 on that seat. However, doing this can result in less people wanting to buy airline tickets and just waiting for last-minute deals. In addition to this, if one airline decides to undercut its competitors, then the competition will respond by lowering their prices and thus resulting in a price war. In both of these scenarios, companies end up losing on profitability in the long run. Therefore, by having set prices, the customer does not have as much power and it guarantees the airline industry a minimum profit level per ticket sold.