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Because it would be slower and have different scaling requirements than the ones they want.

It's not 100TB of data. It's probably 50 GB of data on each of 2000 servers. Because it's a cache. What is the point of a central cache if it's as slow to access as the original data?

TFA gives numbers closer to 5GB.

I'm no expert but presumably all of throughout, latency, and churn. DNS is approximately a giant KV store where the typical record has a TTL of ~5 minutes.

This is smart, task-specific caching in front of database.