...or thousands of other things based on people telling the truth?

The reason they work is that most people actually aren't dicks.

Does not electronically go through your contact list, instead relying on a bluetooth signal to and from other phones.

Look, doctors and scientists aren't idiots. Though some are forced to say things by idiots, or they become idiots by pursuing political agendas in public office or the media. But the ones who set up these programs are pretty well grounded.

They know that people who are closeted won't reveal their real contacts. If a married man had sex with a man and got a disease that gets tracked, they will say it was a woman they don't know. Trace ends.

It's built into how the system works. Most people who are traced for CV will be pretty honest. Some with hidden lifestyles won't reveal those details, but at least the ones they do can be contacted to see if they have any symptoms and be advised to isolate.

They do what they can. But as long as their is a stigma associated with alternative lifestyles, people will hide them at all cost. It's human nature.

...just in case.

It's like "soda." There's bicarbonate of soda, baking soda, caustic soda, and diet soda.

S. Korea had contact tracing. They'd ask you who you were around. If you lied, you had criminal charges filed. To be tested for COVID, you first had to download an app that would monitor you going forward. They looked at phone records from before. They published the names of those infected and the names of likely contacts that they couldn't locate quickly.

Taiwan had contact tracing. Same business with phones and charges. And phone records. Who, where, when. But they also accessed your medical records for symptoms to see if maybe you needed testing. They'd contact those at your home address. Neighbors. They'd access tax records to see where you worked, assume you had been to work and test there, too.

Privacy? When somebody's life might be in danger, somebody's quibbling over privacy instead of social obligations?

It works better in places with a sense of civic duty. In the US, it'll be harder.

Many who insist that protesters have no right to assembly because of public safety issues--their rights kill people!--will suddenly find a much more substantial right to privacy in spite of public safety issues. And trouble'll be a-brewing if you say "your rights kill people!"