I don't mean to make any moral judgments on culture. Certainly Spain and Portugal have contributed greatly to the history and culture of the whole world, by why haven't they contributed as much on a cultural level? The languages that have garnered the most Nobel Prizes in literature are English, French, and German. There have been 11 Spanish winners and only 1 Portuguese. There have been 657 major French operas, 355 German, 682 in Italian and only 58 in Spanish and just 5 in Portuguese. Philosophy has been dominated by French and German, but even Dutch and English philosophers are more recognized. In numerous lists of composers considered the greatest not one is Spanish or Portuguese.
It is difficult to measure culture, even more difficult (if even possible) is to measure the "contributions" to world culture. Nobel prizes are a horrible way to measure these aspects, to say the least, even if it's only in literature. It'd be a lot more convenient if you reformulate the question to something more specific. For example, regarding the operas and composers this previous answer might interest you; by u/caffarelli .