פירוש על עבודה זרה 58:5

Rabbi Ishmael asks Rabbi Joshua why the Sages forbid cheese made by non-Jews. This prohibition is perplexing to Rabbi Ishmael because even the non-Jews make their cheese from kosher animals such as cows and goats. Since the milk used to make the cheese comes from kosher animals, it should be permitted.
Rabbi Joshua responds that non-Jews use rennet to curdle the cheese. This rennet comes from the stomachs of animals that were not properly slaughtered and therefore the cheese is prohibited.