I would suspect that being able to mark your own Answer as “Best” would unfortunately be susceptible to abuse.
For example, there are definitely some bots that have posted here in the last few weeks. Not many, but some. If bots were also allowed to be able to answer their own question as “Best” this could promote their content/profile and whatever spam/URL phishing that is typically within their posts, which is unfortunately today is very common on websites.
Also for any legitimate forum poster there may be a temptation to possibly “pad” their own Points stats, lets say for instance if it was seen that a certain cheating FME lizard was pulling ahead on the Points scoreboard, then maybe we could “fix” this and catch up by raising and answering a few of our own questions. 😉