Because,
1. The private-key range is unknown, so basically 2^256.
2. Even the public key of address which mined Bitcoin on early days is known (whether because it uses P2PK, revealed due to sign message or create transaction), it's still too difficuly to brute force with algorithm such as Pollard's kangaroo.
3. Serious ethical concern.
4. It's more practical to seek weakness of cryptography (such as ECDSA with secp256k1).