It is pretty rare for a person to ask and answer the same question. It is very common, however, for new users to accidentally add an answer to their own question when they intend to comment. The dialog is useful for them.
For higher rep users, self-answers are rare enough that the extra check in the code doesn't seem to be worth the trade-off. Even when they dialog is presented, it still provides value; even high-rep users sometimes make mistakes.
Consider that such a code change would require:
- One or more developers to update code
- One or more testers to test not only the intended change, but all potentially-affected areas (regression testing)
- Possible management involvement to coordinate schedules
- Documentation updates
Since the dialog is useful for new users, and is rarely seen by anyone else (and provides a small benefit even in those cases), the cost does not outweigh the benefit.