Coinbasetx ID Markle Tree on Witness Commitment

The witness’s commitments in one output of a Coinbase transaction include:
OP_RETURN commitment_prefix|witness_commitment

where commitment_prefix=0xaa21a9ed and
witness_commitment=sha256(sha256(witness_merkle_root|witness_reserved_value)).

witness_merkle_root The root of the Merkle tree, which includes all transactions, takes witness data into account. For Coinbase transactions, you cannot use the actual hash of the transaction due to recursive dependencies. To resolve using all 0:

                             sha256(sha256(E | F))
                                        │
                     ┌──────────────────┴───────────────────┐
                     │                                      │
        E = sha256(sha256(A | B))               F = sha256(sha256(C | D))
     ┌───────────────┴───────────────┐      ┌───────────────┴───────────────┐
     │                               │      │                               │
     A (CoinbaseTx)                  B      C                               D
  000...                         txid_2  txid_3                         txid_4

Why use a TX ID set for all 0s rather than ignoring Coinbase transactions and creating a Merkle route?