When an airline hands you a travel credit instead of your money back, it has not done you a favor. It has converted a refund it owed in cash into a coupon it controls. The money stays on the airline's balance sheet. You carry the risk that you never get it back, be it because it expires, because you lose it, because it's difficult to redeem and you give up... Both of those facts are deliberate, and the language wrapped around them, "flexibility," "credit toward future travel," exists to keep you from noticing which way the deal runs.
Start with the obvious part, the part the airline will happily admit. A credit is an interest-free loan from you to a company that would otherwise have to borrow that money somewhere it costs them. You bought a ticket, the trip did not happen, and instead of returning the cash the airline keeps it and promises you a seat later. While it sits there, that balance is theirs to use. Multiply one forgotten coupon by a few seasons of cancellations and you get a real pile of money the airline is holding for free, indefinitely, with no obligation to pay you a cent for the privilege of holding it.
That float is the boring half.
The interesting half is breakage. Breakage is the industry term, borrowed straight from the gift-card business, for the portion of issued value that is never redeemed. It is not an accident the finance team tolerates. It is a line they forecast. Gift cards taught every retailer in America that a knowable fraction of issued balances will expire unused, and that you can book that fraction as revenue once you are confident the customer is not coming back for it. An airline credit is the same instrument with worse terms and a shorter fuse. The company that issues it is not hoping you redeem. It has already modeled the share of you who will not.
Once you see that, the friction stops looking like incompetence and starts looking like the product. Notice the asymmetry:
Booking a ticket takes ninety seconds and the interface is built by people whose bonus depends on you finishing.
Spending a credit means:
logging in
finding the credit
earning it cannot be combined with another credit
learning it cannot be transferred
learning it is locked to the fare class or the route or the original passenger
discovering that the expiration clock often runs from the date of the original booking rather than the date they made you take the coupon. Delta doesn't even allow you to store your own credits in YOUR account. You need to have a notes file keeping track of them.
Every one of those rules has a respectable explanation offered one at a time. Taken together they describe a system that is easy to put money into and hard to get money out of, which is exactly the shape you would design if you wanted a reliable share of it to go unclaimed.
When a thousand small frictions each have an innocent local explanation and every one of them happens to make forfeiture more likely and redemption less likely, the innocent explanations have stopped doing the work. A fraud control that also happened to make credits easier to spend would exist somewhere. A fare rule that nudged toward redemption would show up once. The restrictions cluster on one side of the ledger because the outcome on that side has a dollar value the company books. You do not need a conspiracy. You need an incentive, and the incentive is that your forgetting is their revenue.
What an honestly customer-friendly version looks like is not a mystery, which is the part that should bother you:
Apply the credit automatically at checkout.
Date the expiration from when the credit was issued, not from a booking that already fell through.
Let people pool credits and hand them to a spouse.
Make the balance visible the way an account balance is visible, instead of buried where you have to go hunting.
Each of those is technically trivial. Some airlines do one or two when a regulator or a bad enough news cycle leans on them. They do not do all of it, and they do not do it unprompted, because the current design has a number attached and a better one would shrink it. A company does not volunteer to give back money it has already learned to count on.
So when the confirmation email calls your credit flexible, read it as what it is. It is a refund the airline talked itself out of paying in cash, parked on terms it wrote, with a clock it set, against the statistical certainty that a known share of you will let it run out.