You could say these sentences without "that" or "those" and I would understand your meaning but you are removing the direct comparison and so are then relying upon me making an assumption. Technically you are reducing the accuracy of the sentence. This would be fine for casual speech but not for accurate writing. Without the direct comparator words you are saying: The human brain is more advanced than a chimpanzee. The US students are different to Japan. We likely know that you mean "a chimpanzee brain" and "Japanese students" but that is not what you have said. You could specify these things and therefore remove the need for the direct comparison words but then you make your sentence less elegant. The human brain is more advanced than a chimpanzee brain. US students are different to Japanese students.