Answering your own question, and then accepting it immediately, basically sends the message that "This is the right answer, and I don't wish to see any more answers."
That's not how the site works. Even if you could accept your own answer right away, it doesn't prevent others from posting answers, and preemptively accepting an answer as correct essentially short-circuits the crowd-sourced vetting and collaborative answering that takes place on any posted question, because many people won't bother to participate if the question already has an accepted answer.
I've seen too many people try this and then get substantial feedback from other users that their premise is flawed, or their answer has some factual difficulties. This is valuable feedback.
Accepting an answer right away effectively puts too much ownership of the question into the original poster's hands, and potentially opens the door for all sorts of abuses, like companies using self-answered questions as proxy blog posts, which obtain a degree of immunity from closing/deletion because the number of initial views is reduced, due to an answer being accepted.