That's the point of having an open world boss. Deal with it.
What?! How is that even a valid argument? If you were actually FIGHTING over the kill then I would agree. But there is no fighting. If even ONE extra person attacks the boss outside of your raid group the boss will insta kill everyone. That makes absolutely no sense no matter how you look at it. The only people who benefit from this feature are trolls and griefers who have nothing better to do.