A concept I have difficulty wrapping my head around is having to register to vote. Why the fuck isn't everyone automatically registered? I've never had to do that in Canada.
Short answer, federalism. States have the right to control how voting is carried out.
But, registering to vote isn't really all that tough. You practically need an ID in the US, and in NY at least, you just check a box when you apply. You also are legally required to register for the draft (somehow lots of people that think a mask mandate is going too far are fine with the government putting you in fatigues and sending you to war, but I digress) and that also registers you to vote in most places I believe. It is also a legal requirement to offer people the opportunity to register at the DMV when they get a license or whatever else, although that doesn't always happen.
I think people not being registered is often an issue of not being registered in the correct place. Also, keep in mind the electoral college discourages many people from voting or registering. Despite the fact that I had very strong opinions in this election, and actually donated some money to support a candidate, I didn't care that much about actually voting. Because, I live in NY. Biden was going to win pretty much no matter what, and if NY was close enough that my vote would matter, then Biden would have lost in a landslide anyway.
I honestly don't care all that much. As long as people have the ability to easily register to vote (which they may not), it's a non-issue for me if they don't choose not to. I think voting is a right that should only be exercised if a person has informed themselves on the issues at hand. If it's not important enough to someone to spend the time to register, I'm fine with them not voting.
That being said, we should take steps to encourage people to be engaged, and to be willing and able to make rational decisions... but that's a much larger problem.