I think it's mostly because of the beauty standards which tends to glorify skinny body types over curves. That's why no one sees problems with gaining weight as problems, cause they consider it as something positive. It's actually pretty sad, but I hope you'll find a way how to gain weight you want to :)
I think that trying to gain weight is a less common issue and not something society sees as a problem. Most people tend to want to lose weight, so they would are almost jealous of an inability to gain weight. Because it goes against what the majority want, trying to gain weight is not talked about. However, it can be just as difficult as those trying to lose weight. It can be especially difficult because attempting to gain weight is not seen as a valid problem.