I think that the reason you can't do this is the same reason why I can't accept my own answer for a few days after I ask the question: the question has a lot more attention (by which I it's displayed in a specific tab) before awarding the bounty, and you might just find a better answer on the way. Additionally, the extra attention will likely find the answerer with more upvotes to their answer, which rewards them even more, which is the whole point of exemplary-answer bounties, right?
Don't get me wrong, I wanted to award one of my own bounties instantly too, but this is something that we need.