How many times have you actually interacted with the police? Between being pulled over several times, living in an apartment that was previously occupied by a guy with warrants, being assaulted with a golf club, and various other stupid minor run-ins, I've had exactly one experience where the cop wasn't a complete dick and possibly would have let me get halfway through that wall of text you want to preface your lack of interaction with.
It's just naive to expect that you can, as a rule, calmly explain why you don't want to talk with them.
Saying "I have nothing to say" works. Saying "May I go now?" works. Trying to justify yourself, using phrases like "I can't" or "I don't want to", implying that you might have something to say if a lawyer were around, anything more than conveying you have nothing to say and want permission to leave, is just asking for a headache.