Accepted Solutions are used to calculate reward levels and therefore you shouldn't be able to tag one of your own replies as an Accepted Solution.
There's a user that received one of the top badges this month that started 7 (mostly inane) different threads and accepted one of their own replies as the solution in each one over the last couple weeks. This allows users to game the rewards system and make themselves look more helpful than they actually may be. It's dishonest IMO.
If people can't be ethical on their own then PM should remove the ability to accept your own reply as a solution to your own post.
Sorry if this idea sounds petty. Kinda sad that I have to even suggest it to be honest.