If you're in the US or Canada, driving for 10 hours means you're going to visit your aunt. Doing the same anywhere in Europe means you're likely 2 countries over, and it becomes a 'huge trip' in people's heads. Heck, I know several people who have driven more than halfway across Canada, in Europe no-one who drove e.g. from Paris to Istanbul.
And while the European market is supposedly unified, in practice there are significant variations in pricing and availability as soon as you cross a border, simply because that's something few people do. And of course the language difference means most local media is opaque to outsiders.
So I would say that the US is definitely diverse, but it's a gradual diversity, that transitions smoothly from ultra-urban to no-one-for-miles rural. It works on very large distances, and is mediated by a shared media, politics and language. European countries meanwhile are much more homogenous on the inside, but there are a lot of forces keeping each one unique.
Abstract away the vast distances of road and think of it as nodes on a directed graph. How many nodes (ie: inhabited locations of distinct identity) did you pass through from source to destination? That's a kind sociocultural or even ecological distance you traveled, and it can actually be larger on a Paris-Istanbul trip than on a cross-Canadian trip.
These differences between regions inside one country is exactly what is comparable to differences between Midwest and Northeast.
Now think about the fact that Europe consists of tens of countries, many of which are significantly more diverse internally than Poland. And try to compare this monstrous complexity to differences between USA regions.
For me it seems to be daunting and impossible task because of huge difference in scale, yet you have no problem with it. I can only admire your courage :)
That is why I think that despite existing notable differences between those regions of the USA, they are on a different scale. Dialect, culture, food, general behavior, economy and (even) quality of life, of each country is affected by it's history.
There are 5 major regions of the US: Northeast, Southeast, Midwest, Southwest, and West. Each of these regions has noted differences in: dialect, culture, food, general behavior, economy, and quality of life.
Now regardless of US or Europe we have to factor in the population density of a location. It's no secret that urban culture is very different from rural culture be it in Oregon or in France.
Granted: we mostly speak the same language, we all share a common currency, and we all answer to the same federal body which represents us.
There is more diversity in Europe, that much I am sure of. But there is no problem with a comparison of the two.