Why do people judge you by ur last name?

i mean i always hear ppl say "you cant hide ur last name " or w/e as if ur name tells someone ur race or personalty. i mean if ur last name is gonzales what does that tell me ? nothing.they could be white from argentina or black from cuba.plus by that logic all black americans must be British since they have names like walker,smith,harrison. get what im saying. i can easily say that Filipinos and cubans are the same because they both have spanish names =] lol but ppl will laugh at me.but if spanish=hispanic and many Filipinos have spanish names then.....this further proves how u cant label a 2nd 3rd generation by their name.you dont do it blacks why do it asian americans or ppl who have names like goldberg and bernstein but are atheists and speak only english not hebrew.