My guess would be insurance. It costs a lot more to insure passenger flights, which means that the tolerance for failures is that much lower. I.e.: it costs a lot more to insure an old piece of junk than a brand new piece of kit.
Brand new vs paid off makes no difference on fuel efficiency. However, the average car payment for new cars in 2023 is $716 (holy crap!), so that payment for 5 years ($43k) is going to far outweigh the $150/year you'd "save" by driving a more fuel efficient vehicle. Five years of the fuel for this 15mpg truck example is still only $750 more than the cars fuel, a little over 1 monthly payment. An older vehicle is going to have higher maintenance/repair costs but not $40k more.
I assume larger numbers, but similar ratios for planes doing short daily trips.
At that point, if you got the truck dirt cheap, it doesn't matter how fuel inefficient it is.
The second biggest cost is lease, which is significantly smaller the older the plane is
(also cargo doesn't fly first class)
A lower insurance (which is my guess/ opinion!) and a lower lease cost has a greater explanation value.
If you can buy something for very cheap, and only need to use it infrequently, it isn't a huge problem if it's inefficient for that use a few hours per day.