You're right that human contact-tracing also has false positives/negatives. I think a better question is how does the error rate compare? The worry is that auto

You're right that human contact-tracing also has false positives/negatives. I think a better question is how does the error rate compare? The worry is that automated exposure tracking will pick up many more false positives than humans will because it's hard to judge how likely a transmission occurred. A phone doesn't know if there are barriers between people (walls, PPE). The downside to a higher false positive/negative rate is that you'll create more false alarms. At some point, people will start ignoring false alarms. With phone tracking, you are probably trading off many more false positives for a couple fewer false negatives. It's unclear whether this tradeoff is worth it because we don't know how people will respond long-run to automated exposure notifications.