I have a question with a bounty that will expire in a few days. It looks like nobody will get the bounty; only one answer was posted during the bounty period, and it has no upvotes. In cases like this, the bounty is thrown away. Why isn't it refunded instead, if no answerer has earned it?
A bounty is not a guarantee. It's less risky in the new system, but it never has, nor ever will be, a guarantee for the answers you need. It's payment for a bid for an answer, not payment for actual answers themselves.
Remember that although you didn't get an actual answer, you did get 7 days on the Featured tab. If you got refunded, then you could keep your question on the Featured tab forever, which really reduces the benefit that placing a bounty has in the first place.
Basically, consider this like a parking meter - the reputation you spent is good enough for only 7 days. If you need more time, you need to spend more reputation. Of course, it's a parking meter where you can pay however much you want and get the same amount of time, but the extra payment is for the possibility of better quality attention during that same period.
An excerpt from my answer: