I'm skeptical of your claim that young people are actually using desktops in those situations. As far as I've seen, young people are primarily gaming on consoles and are using stripped down devices like Chromebooks at school. It's also been noted that computer skills have gotten markedly worse among young people in recent years, which suggests they are not in fact being exposed to desktops regularly.

And I definitely think you're off base about mobile devices being popular because they're mobile. People will sit at home and use a phone that is an incredibly worse experience than using an actual computer, and think it's an adequate substitute. They don't need the mobility in that situation, but they really think there's nothing wrong with the user experience.