Because the miles are awarded to those who fly - not to the person that buys the ticket.

These can, indeed, be two different people.

Only the person who actually flies, will get the airmiles. And the airmiles are awarded contingent on actually taking the flight. The airmiles are not awarded on the basis of money having being handed over. If it was, then the airmiles should be credited to your account as soon as the funds clear; and the miles would be determined by the amount of money paid, not the distance that will (eventually) be flown.

The agreement is always that the miles are awarded AFTER the flight has been taken.

For those who change plans or who fail to travel - no miles will be awarded. Similarly, you will not receive your on-board meals, free drinks, or a guaranteed seat to your booked destination, if you fail to turn up for the flight. Just as is the case with the miles, you cannot merely request these things you have "bought" to be delivered to you anyway.

If you are entitled to a refund, you should ask for one.