why do white people call them asians? there are thousands of different cultures and countries that are "asian" grouping them together is retarded

Because white people in America group themselves like this. Unlike in European countries where people have shared ancestry and history, we are a melting pot. American whites are often of very mixed heritage (me included), so their defining category comes to be 'continental' origin rather than 'country' origin.

Also Asians do share characteristics besides their continent. They all have epicanthal folds on their eyes, and speak dialects descending from chinese. Its really just the Indian/Pakistani/Bangladeshi's that are not actually 'Asian' in the way that most Americans understand the meaning of this term

Because we are such a melting pot, our categories in America just became more broad than elsewhere