The secure enclave is not a general-purpose/user-programmable processor. It only runs Apple-signed code, and access is only exposed via the Keychain APIs, which only support a very limited set of cryptographic operations.

Presumably latency for any operation is also many orders of magnitude higher than in-thread crypto, so that just doesn't work for many applications.

If you look at the cryptokit API docs the Secure Enclave essentially only supports P-256. Which is maybe why they didn’t include ECC crypto in the examples.