They do know what they are getting into.
But they either focus on the positive story's and previous islanders who are liked and receive very little/zero abuse. They assume everyone will like them, because everyone they know does.
Or the thought of fame is so enticing to the them, they ignore the potential pit falls and risks. And they arent honest with themseleves about what they can mentally handle. They assume they will be fine.
Nothing in those articles come as surprise. I know I wouldnt be liked on there. Because I am not a huge fan of socialising anyway. Plus my anxiety would mean I would spend most of the time thinking about what's happening out here and what people thought of me.
I used to work for british gas and they were talks about doing the 'call centre' show. I was managing an area they were thinking of using. Even then (in my twenties), I knew I didnt want to and told them so. I am not the reason they decided not to go ahead, they would have just moved me. But the though of being a character on a reality TV show made me feel sick. Because of the things producers and editors do.
For people who go on them, they seem unaware of themselves and the benefits outweigh the risks, to them.
But then they come out and realise the risks werent worth the benefits.