How do you contact someone's contacts and follow those tributaries and follow those and follow those and follow those?
How would you know the names and how to contact them? You're in a huge crowd of strangers and get sick. All those there have the potential to have gotten near enough to you to breathe in your droplets or whatever. and get sick or become carriers. Unless you KNOW everyone there and how to reach them where does contact tracing go? Needle in a haystack? If you miss one contact it may have lead you to hundreds of others. You can't know whom you didn't follow through with so what happens to those you miss?