@Donald28 sure that would be nice. Also, why don't they give us $200 every time? That would be nice too. 🙂
As with all coupons, etc., the company's goal is to make you feel good with as little actual cost to it as possible. If you know the coupon will expire soon, maybe you will use it to cover part of a trip you wouldn't otherwise have made-- and Airbnb will still make money off of you! But if you save it up for a trip you would have made anyway, and they have to provide your entire accommodation for free, that's a real cost.
Brian Chesky is not your grandma.