Because the cost of the plane is dwarfed by the cost of fuel, maintenance and crew.

Seems a new plane can cost upwards of $80 million. A good condition 2nd hand one $6 million.

Let's say an airline utilizes it's new plane on 10 legs per day. Over ten years that's 10 times x 365 days x 10 years = 36500 flights.

Divide the cost of the plane per passenger per flight: call it 200 seats for easy maths although most 737s for example have less.

So 80,000,000 divided by 36500 around 2,200 USD per flight is the cost of the plane. Divide that by the 200 passengers means the plane is around 10 USD of the flight; even if there are a lot of empty seats it might be around 20 dollars of the flight ticket.

Compare the 20,000USD it might cost for the fuel per flight. This equates to 100USD per passenger, a much higher factor.

Remember that using an older plane will increase maintenance costs, and should one ever fall out the sky would likely be the death knell for the airline too.

I got the rough figures from this article:https://www.telegraph.co.uk/travel/travel-truths/how-to-buy-an-aircraft-boeing-cost/