In all other games (minus Game Duels but we'll get to that) you have more than one opponent. The odds of 11 people all leaving so you get an instant free win are... rather low. If one person leaves that's perfectly fine because another 10 are waiting to kill you, you'll still deserve that win.
I do get what you're saying but you can't compare other games to Duels due to the simple fact there's so many more people in those.
As for Game Duels, they're different aswell because there's a void there, in Lucky Islands it's also very easy to simply get an unlucky Lucky Block and kill yourself on accident. There's also additional items which don't give you kills either. Overall the amount of people that accidentally die, or are killed with weird items, outweigh the people that intentionally die or leave in Lucky Island Duels. The same cannot be said for normal Duels. Additionally it's entirely possible to kill people several times in Eggwars so that definitely wouldn't work and can't be compared.
As I said it would be lovely if there was a way to make Crystal PVP actually give you the kill, although I'm not sure that's possible. Even if it isn't however, I still wouldn't support this suggestion since this only affects one single kit, with one single item, whereas changing the entire leaderboard affects every kit, every person, and just seems unfair at this point. I'm willing to bet you'd be gaining more wins from people leaving than from playing Crystal PVP, unless you're intentionally selecting the kit that is.