Way more people claim to have Spanish ancestry in the US, since most of the Latin Americans/Hispanics can trace their origin to Spain (some having mixed with Amerindians), and in places like New Mexico, Texas or California, you had Spanish people before the US annexed it (see https://en.wikipedia.org/wiki/New_Mexican_Spanish).
These people aren't claiming ancestry from spain, since the meaning of "hispania" isn't widely know. Rather, folks who claim "Latino/Hispanic" ancestry are identifying with mexico, central america, and south america.
Yes, technically if you identify as "Hispanic" you are claiming ancestry from spain, but this isn't what most people mean when they use the term in america.
They can claim to have foreign ancestors (like most of the Americans), but most of them aren't Irish, German etc. Most of them lost any cultural trait from their ancestors, they ceased to speak the language, ceased to have the cultural references that once their ancestors had. I agree with you though about the Hispanics successfully retained their culture, but they are an exception in comparison to other minorities such as Germans, French, Russians, etc.
See my point about religion above. The same can be said for most Irish americans, who remain mostly catholics.
It's not just faith. If you are traveling between different regions of the US there are differences in accents, food, politics, and the way people act in general, all owing to different cultures.