Speaking only for myself, as a young person who does not vote, it's not a lack of interest in politics or the state of world affairs. I'll vote only once the electoral process is significantly reworked, until then I have better ways to engage civically.
I bet most of them are people who don't care about politics. Good luck trying to engage them, people have been trying to do so for decades and have failed. The simple reality is that life is comfortable enough for most. Few are starving, few are persecuted- if you're earning enough to live a relatively comfortable life, why bother voting?