Because we don't teach people ANYTHING about the political or legal system here (and legal matters a lot, since it's intrinsically tied to how we make and enforce laws here).
For instance, just figuring out where to vote, when to vote, and what you need to bring (and legally can't bring), varies WILDLY state to state. Real examples:
I lived in state A, where the primaries were just done with a ballot, you physically had to show up at a specific day and time to fill put in person
Moved to state B, where the primaries were at a different time and were done as a caucus. Meaning I had to physically go and stand there as my body was my vote. They were unprepared for voter turnout and it took 6 hours to vote. There were almost no chairs. Handicapped people, elderly people, and people who had to work, left. Like they just couldn't vote. It was horrible to witness. That's our rights being denied.
And also at state B, you had to be registered in advance and they needed to have record of it. If there was some weird error where you weren't on the list, tough.
Moved to state C, where everything is super easy and done by mail. Just mail in your ballot. No big.
We need to take kids on field trips to vote and teach them actually how to do it all. Including looking up smaller election dates. It's not actually that easy or that streamlined.
We need to take kids to town halls to show them how to observe public meetings. We need to show them how to run for office. The public deserves a clear and thorough education on this, as much as English or Math.
Here, we have mock elections in school, held with the same booths that the real elections are done. The kids make up political parties, then they go through the whole process, from getting their voter card (informing them about the vote and where the vote takes place or how to request vote-by-mail) to voting to counting and having results.
There is something called student government but it doesn't actually mimic what happens in each kid's district. They do not make up parties. They mainly just vote for class president, secretary, and treasurer with a show of hands. Sometimes a ballot. That's it.
They do not do any of the rest here. I'm being VERY sincere when I emphatically say most states do NOT teach the public ANYTHING that will give them legal or political power including how to vote. And some states make it very hard to vote.
Yes, unfortunately that's not a priority in the US. You know what is though? Making sure we are automatically enrolled in the draft, and that we take our ASVAB (military test) in high school. And of course, every high schooler knows how to enlist in the military and all the amazing benefits they could get.
Really? I didn't know anyone during high school that would've known how to enlist / benefits. It really depended on whatever anyone was interested in, what people knew about.
I grew up in New York City.