Simply because it is not completely true. Most of those who present Italy in this way are usually non-Italians who tend to misunderstand the behaviors and ways of Italians. It remains true that, especially in the past, theories have been devised, even with a racial background, which tried to justify the backwardness of the south by defining the Italian south as another race, but also in Germany the population was divided into different races (Alpine and Nordic), but this is a borderline case that goes beyond what most people think. Italians tend to highlight their regional peculiarities and this, abroad, is taken as confirmation that there is a lack of a sense of national unity which, however, exists and is present. And this sense has been present since the Middle Ages, Dante, father of the Italian language, spoke of Italy as a political and cultural concept, Machiavelli in 1500 wrote essays to explain why Italy was not politically united like France and always in the 16th century Italian intellectuals debated what should be the common Italian language to be used in all Italian states, a common language in which books and laws would then be written and which would be used in universities. I could go on to give thousands of examples, but the fact is that many think that the concept of the Italian nation came out of nowhere in the 19th century, while it was already present in the minds of much of Italy.
I note that this reference to divisions is often remarked in the youtube videos of foreigners and Italians who have a superficial culture, often obtained from the vision of so-called videos. I can assure you that both at the popular level, as a man in the street, and at the academic level, in the universities, no one will tell you that the internal difference between Italians was so greater than the German one.