Bounties were created as a way to in some way reward those who have put a lot of time and effort into the site. They are able to spend some of the good karma they earned in order to get a question answered.
Creating a sliding scale percentage option does not really make sense, because each bounty is created according to how much the user is willing to give, and as they gain more and more reputation, it is more likely that they will offer more. Creating an enforcement of the percentage is just creating extra rules for no reason. The users who already did approximately that will not notice, and the users who don't will be upset. Everyone loses.
An additional bonus for creating a bounty is that it gets the question on the featured page. There are bounty hunters who troll the feature page specifically to gain the generous extra points. If the bounties were to get smaller, then the "featured" page would be come diluted and less visited. It would be covered with tiny, inconsequential bounties which no one particularly cares about.
Furthermore, because of the rules associated with the bounty. It would mean that after seven days, an "answer" would be forced upon the question, which would be unfortunate if the answer was wrong.
Ultimately, I think we should leave the bounty rules the way they are.