Why are Clifford gates alone not enough for universal quantum computing?

Clifford circuits act only on the computational basis states, so non-Clifford gates are needed to create any superposition at all

Clifford circuits can be efficiently simulated classically under the stabilizer formalism, so non-Clifford resources are needed for universality

Clifford circuits cannot create entanglement between qubits, so a two-qubit non-Clifford gate is required before any correlation can appear

Clifford circuits cannot be implemented fault-tolerantly, so non-Clifford gates are added to make the encoded operations transversal