1) Voter ID laws disproportionally target younger (and older) voters. In many cases, a college ID is not an acceptable form of voter ID. If a student goes to school out of state and has a drivers license/state ID from their home state, that is not valid if they register to vote in College Town. In most cases, Grandma (who gave up her license) can get an absentee ballot, but young people need a reason to.
2) Civics education is lacking. I graduated in 1998 and was taught how to vote (not who to vote for, but a voting machine was brought in and we had a mock election) by the board of elections. I'm not sure if schools do this now.
3) They feel that politicians are not listening to them or the issues that matter to them. This can be solved by electing younger people to office that understand the issues that the millennial generation (and Gen Z behind them who is now coming of age) understand-- things like the cost of college and student loans.
4) Some of the rabid rightwingers are pretty vocal about younger people not voting ('oh you wouldn't understand' and 'they should go back to only allowing white male property owners over 21 vote'). I see this in my own family. Some feel that only established members of the community should vote