Why is error correction difficult in quantum computing?

Quantum states are fragile, so codes must correct bit flips only, since phase flips cannot be detected by any measurement

Quantum states are fragile, so each logical qubit must be measured directly and repaired between every pair of gates

Quantum states are fragile, cannot be copied perfectly, and require many physical qubits to protect logical information

Quantum states are fragile, but errors are discrete, so a single spare qubit per logical qubit is enough to detect them