To be fair, I'm pretty sure traveling from Washington State to Florida would surpass the length of Europe. I'm also not gonna do the math. Because lazy.
To continue to be fair... I dunno. I guess it's partly because it's expensive, partly because xenophobia, partly because fuck if I know. But I'm fairly sure it'd be easier to get, say, from france to Spain, than it would be to get across the Atlantic from the US. In the US, Canada just is not portrayed as an exotic tourism destination and friggin tourism boards and travel advisors all say don't go south of the border because drug lords will kidnap your shit, so there's somewhat of a lack of motivation to travel to other countries on the same continent.