Getting an ID in the US is not as simple or cheap as it can be in Europe. A lot of people simply can't afford it.
Most people don't travel abroad (expensive) so they don't have a passport.
Usually the drivers license and social security card (not valid for id on its own) is all they have. Even getting a copy of your birth certificate can be time consuming and expensive.
Everything is done on paper. You need to go to the court house and wait in line during the day when you need to be at work (and can't get time off). You have to update your registration every time you move, even if it's a few blocks because of gerrymandering. Your voting location changes and is not always intuitive. Parking can be limited and lines long.
You need paper copies of multiple other forms of id. And then they can make you do it all again by taking you off the voter list for "reasons".
This is all intentional. They don't want poor people to vote.
This is on top of the combo of a complicated electoral system that honestly most Americans don't even fully understand, partially because it's not taught well in schools.
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