This is somewhat obvious if you realize that HTTP is a stateless protocol and Anthropic also needs to re-load the entire context every time a new request arrives.

The part that does get cached - attention KVs - is significantly cheaper.

If you read documentation on this, they (and all other LLM providers) make this fairly clear.

They really should have just set the cache window to 5:30 or some other slightly odd number instead of using all those tokens to tell claude not to pick one of the most common timeout values