Many players have families they'd rather be around during those two weeks of summer training, especially since they just spent a long season involving significant time away from their family. While the idea of a mini-vacation in order to play hockey representing my country (though good luck if the US women's team ever selects me, they'd be doomed) greatly appeals to me personally, it doesn't appeal to everyone. There are surely some players who don't have a wife/girlfriend (or kids to go along with her), but while I'd imagine that would make them more likely to accept an invite, surely there would still be some among that type of player who still wouldn't want to. It's their personal decision and right as a human being to decline, which I'll respect even if it means bad news for my national team.