Acceptance expresses the preference of the author of the question. This is the only purpose of the checkmark; if the questioner happens to prefer their own answer, it's their business. Artificial barriers around expressing their preference would make the checkmark less meaningful.
It should be noted that one does not get any reputation for accepting own answer.
Votes, on the other hand, are meant to be some (very rough) measurement of the usefulness of the post. Allowing users to rate own posts as useful / not useful wouldn't make this signal any more meaningful: of course everyone considers their answers useful, otherwise why write them?