Well each US state is more-or-less autonomous compared to the Spanish regions. I guess in America states don't, by and large, correspond to the cultural regions

Well each US state is more-or-less autonomous compared to the Spanish regions. I guess in America states don't, by and large, correspond to the cultural regions that could become independent. Also, the movements demanding autonomy predate the autonomy in almost every case (where they subsequently demand independence). Other times, perhaps the reason for the autonomy demand is not national difference like in the Spanish cases, but simply distance - therefore, it doesn't become a movement for independence. Not only is the autonomy sufficient, but perhaps a backstep is taken: For instance, in mid-19th century Australia the Port Phillip District of NSW demanded autonomy and received a parliament that could do almost anything domestically. Fifty years later, they began reuniting in a federation and there's been not even a hint of independence from Victoria since. Ultimately I think the reason for separation demands in Europe is because Europe is now the federation level of government, and the regions (Flemish, Catalans, Scots) want a seat at the table. So they're not really demanding independence but separation. This is a normal consequence of forming a federation.