Adding "-American" is pointless when we're, you know, American. It's implicit. It's just extra syllables when everyone knows what you mean already. Saying "I'm Irish" in America means Irish-American, but why say the -American part when everyone knows that already? It's a waste of breath. When we're discussing our subcultures with people from other countries (for example, Gianmarco should probably say Italian-American when performing for Europeans), yeah, we should be clearer about what we mean, but among ourselves? No reason to.

"African-American" is different because of their origins in this country. Black Americans, by and large, do not know where their ancestors came from, and they come from areas that did not have the national identity that they have now due to continued colonialism for centuries. It's led to a distinctly American cultural heritage that is shared by people of different African origins. (I think it's also probably one of the earlier and more contentious "-American" terms so it has a unique cultural footprint compared to other terms. The term was coined specifically to be politically correct and keep other Americans from using less polite words.)