You have to remember that up until that age they have very little control over their lives. They are just starting to get the power to make decisions and most of them are still heavily influenced by their parents' wishes. It's very easy to feel like the choices you do have don't matter on a grand scale. Their measure of success are usually individual heroes, people that excel in a sport or artistic endeavor, not a communal effort the way voting is. And there is a "both sides" narrative that is very insidious, even amongst older voters (and it's not 100% wrong when you dive into certain issues). So it's pretty natural given recent history and our cultural leanings why young people feel like their vote doesn't matter.