That's not entirely true. Superman hides his true name when he's in disguise as Clark Kent. Plenty of people know his name is Kal-El. Some even call him Kal. Also, while he started out as Krypton's sole survivor, he's no longer the last of his kind. There have been plenty of Kryptonians who've showed up over the years, including his cousin Supergirl, in a couple of versions. The rest can be true of a lot of characters who are heroes.