I think the big reason is…if they could find you, they’d just come and get you.

Smartphones make it a little bit more possible to actually be unfindable while still technically contact-able, but I believe police departments can still trace you through them. So if you’re in a situation where they can send you a message, they can just come and slap the cuffs on.

Also, you’d have to actually enroll in whatever notification system they set up, and what are the odds that anyone (who’s actually likely to ever be notified) is going to sign up?

And then you get into the problems of privacy (what if someone else enrolls their phone number under your name?) and consistency (What if you change phones or move? What if you get banned from your email provider?)—a lot of crime (well, the kind of crime that the police actually pursue) is committed by people without a steady address; in fact, that’s part of why they’re committing crime. Besides, the reality is that a lot of jurisdictions keep very little information about you on file: your name and address, maybe a phone number, and that’s it. Sure, they could find out more, but there are a lot of governmental entities that consider the postal service the only valid means of communication with citizens.

All of that is a big bundle of trouble with no real upside. If you did something wrong, you probably know to expect a knock at your door.