Earlier quoted context omitted.
Why would you bring a mission to a hostile island otherwise?
Because you think god is immoral, and you want them to meet his mercy?
Because if you have civilization imperfect eyes are not a death sentence.
Because you can actually survive your 30s (odds > 1% or so) in civilization.
Because of music, movies, culture, science, ...
Oh and if you must have the more practical reason: because the priests come from the cities, usually a while after the city got brutally attacked with every man (and male child starting from ~puberty) from the village and even a few women trying to indiscriminately kill anyone they meet with spears and (before) poison and forcing everyone else to shoot at them, to devastating effect on both sides? Because such a priest sees the bodies, hears the stories of everyone "they'll try it again in 10 years", of mostly villagers but also a number of people that just happened to be in their way, and wonders if there's another way? Oh and, by the way, for this particular purpose, "our savior Jesus Christ" IS in fact another way and a solution, and that, of course, is how religions work.
Because "right of the strongest", brutally imposed through massacres and rape, is not in fact moral, I don't care what ideology or religion you subscribe to? Subsistence farming is a short, brutal way to live and this way of life dials that up to 11, or to 500, really.
I'm not sure why anyone would either not know this, or even insult it, as if the success of your ideology, of "natural purity" is somehow more important than kids surviving, and all the other reasons mentioned above, and I'm sure you can come up with 500 more.