Unfortunately, Italian teams do not have much money, in fact they are practically up to their necks in debt, and it is difficult for them to compete against other European teams that can spend crazy amounts of money to have the best players available. See Real Madrid and Manchester city, they have practically 22 champions in their squad and no mediocre ones. In contrast, Inter, Milan and Juventus have few top players and many good players, but in recent years they have still been able to fight to win a European trophy
This is a global problem for all clubs in Serie A even if they are big clubs but they do have financial problems which makes it a bit more complicated for Serie A but on the other hand they have always performed above average when compared to clubs in other major competitions such as the Europa League, Champions League or Conference League despite their financial constraints.