My question is very obvious, Why can't I delete my own thread?
Because you can not delete your own thread/topic. The admin disabled it for members. Obvious reason to stop scammers or people who want to delete the evidences. Deleting a topic will also delete all the comments on a topic which is kind of hurting for the users who took their time to comment on a topic.
mdayonliner already stated the obvious fact why we are not allowed to delete our own threads.
But if you just created a test thread that you need to be deleted, then you just rename it as "To be Deleted" , change the content of post as "Deleted" and press "Report to Moderator" and fill "test thread , need to be deleted" and submit. Mods will take care and delete it.