And why can't an arbitrary set of genetic and cultural traits have a name? Everything else in this universe can be broken down to an arbitrary set of something. Why for e.g. call somebody an American when it's only a person living withing arbitrary lines on map?
I'm sorry but that is a really weird argument. Who are those groups of people who want us to pretend there are no similarities or differences between them and other groups of people?
Calling it "pretend" is both dismissive and begs the question. The people you want to call "Hispanics" are ethnically rather diverse, far from homogeneous. The similarities you see originate in ignorance. Mexicans aren't Columbians, Brazillians aren't Portuguese and all of these countries are home to many different ethnic groups with their own histories, languages, cultures and genetics.
I'm not calling anyone anything. The GP of this post claims to be of Hispanic race. You claim he is not entitled to call himself that. Are you the the arbiter of what he is allowed to call him self? I'm must admit I do find it ironic when you simultaneously claim groups of people don't want to be named and then at the same time are trying to prevent somebody from such a group to call himself that.