It's because they, like you, get nothing out of revealing information that they aren't required to reveal, except they have training that says, "Don't say anyth

It's because they, like you, get nothing out of revealing information that they aren't required to reveal, except they have training that says, "Don't say anything until you have to, even if it seems rude," whereas the average person stopped does not treat it quite so adversarially, and tends to waive their right to remain silent. Presumably, the reason that so many Kafkaesque stories crop up is because these people are not required to tell you why you are being detained if asked.