Here's the thing: Sweden has < 10 million people. And not all of those people can vote (children for example). On the other hand, the United States has a population of > 300 Million people. And ~180 million people are eligible to vote if they chose to do so. Then again, we consider a 50% voter turnout good. That's 90 Million ballots to count up. In the same amount of time as it takes you folks to count up 6-7 million ballots. Clearly (to me at least), due to the sheer number of people, different systems are needed here.

Or you could look at land size. Sweden's size: 449,964 km2 or 173,732sq mi. United States size: 9,826,630 km² or 3,794,066 sq mi. Have to transmit the data somehow - its not like you can drive the ballots across the US to one central point in under a few hours. So why not do it one at a time as they're cast electronically? Assuming it's done properly at least.