Perhaps because people intuitively understand that "all X are Y" really means "most X are Y" or possibly even "a lot of X are Y". In other words, that the phras

Perhaps because people intuitively understand that "all X are Y" really means "most X are Y" or possibly even "a lot of X are Y". In other words, that the phrase "all X are Y" does not imply "A is an X, and therefore it's also a Y". Generalized statements are known to be generally more inaccurate than statements about specific objects.