Because that's not how the accounting at Airbnb works. Airbnb holds the guest's money until the time of their reservation - then they pay you.
https://www.airbnb.com/help/article/425/how-do-i-check-my-payout-status
https://www.airbnb.com/help/article/165/how-do-i-refund-my-guest
If the guest is only getting 50% back, you must have chosen a cancellation policy that would pay you 50%. That means, when the guest's now-cancelled check in date arrives, you will be paid your 50%. Since you offered the guest a full refund outside of the policy, they will pay him today, adjust your next payout and then pay you later for the amount you are owed due to the cancellation policy. This is why cancellations and refunds are a big deal for Airbnb - they have that money working in their pockets right now, have a payout schedule with projected earnings on those monies and a refund disrupts that so they make you pay for the refund out of your pocket now so they don't lose anything - as a matter of fact - they get to make more money because you refunded the guest. Capitalism at its best and, thus, why they have a $30B valuation