>>30848681
It's an 'ultimate' ability. I'm guessing you have probably never played with a Planeswalker, but it's not easy to get to use a Planeswalker's Ultimate, as a result they tend to be big flashy things that either win you the game on the spot or make it very very hard for you to lose. Karn is 7 mana, that's not easy to achieve either, and takes him 2 turns without being interfered with to Ultimate. If you try to Ultimate him as early as possible then chances are you're only going to restart with say an extra land in play, while that is pretty fucking good it's not exactly an auto win. The way you use Karn is he normally hits the board, -3's and then you can start ticking him up and sculpting the perfect exile pile before you restart the game, at that point his Ultimate basically reads "I win the game" and your opponent will often just concede immediately.
tl;dr Yes, it sounds broken, but you need to do a lot of shit to pull it off correctly.