That's a correlation, but the causal relationship is the other way around. Unions form after a country industrializes as a response to income and wealth inequality. So you're comparing the income and wealth of already industrialized countries which subsequently developed trade unions with presumably countries that haven't industrialized yet.

Indeed in the Scandinavian countries you're citing, excessive trade unionism, among other illiberal measures, in the 80s and early 90s led to economic crisis. There were economic reforms in the mid 90s which restored economic growth by paring back the role of the state and trade unions.