1) They currently do not have an element of choice. And it would actually be very difficult or impossible to proportionate them a full choice since they would have to know what their lives would be like if outside the island, or with a modernized island.
2) Human beings fear the unknown (quite rightly so, often).
3) You can make the relativity argument all day. I personally believe that a person that is confined to an island in pre-agricultural civilization does not live a life as fulfilling as one that can access vast amounts of information on the internet, can read thousands of texts written over a span of 3 millenia on every topic imagined, can hear Antonio Vivaldi [1], almost void of unnecessary or unusual suffering and privation, for over twice the lifetime.
[1] https://www.youtube.com/watch?v=Z39KxbbtCC0