It wasn't so long ago that the Catalan language was banned. People who were alive then are still alive today. It's more convenient to buy an umbrella on a sunny day. If they get independence today, they can spend years building up the strength they need to keep out those who would try to destroy their community later on. To describe anything as "historical grievance" that isn't happening right now is naive. It hasn't taken long for formerly second world states like Poland and Hungary to prove that democracy rule of law, even in the EU, aren't things that you can count on for decades to come.

In first world European cases, though, I think it's much less about independence and far more about being a top level subdivision of a country. Catalan, Flanders, Scotland etc want to become member-states in the EU with a seat at that table. They're after a seat at the table that decides the rules that govern them, and want to be able to defend their own interests. This is, after all, the point of a federation, so it's a natural thing to want: If it weren't a valid desire, the heads of government of the member states would not have a role in the EU. It's fairly common for territories to be cut up and distinct parts removed once they join a federation, especially when they're at least as large and diverse as Europe (take a squizz at India's states).