First, because both of those steps are optional: the seller can choose whether to keep bitcoin or local currency, and the buyer can choose whether to keep bitcoin in stock or buy it as needed.
Second, because convert-transmit-unconvert still bypasses many of the painful aspects of other money transfer mechanisms. Both buyer and seller only interact with entities they trust (coinbase and their bank), no credit cards (and associated fees, chargebacks, and headaches) get involved, and the transmission process in the middle uses bitcoin in lieu of ACH or similar (avoiding many potential risks, reversals, and the various Bad Things that can usually happen if you pay directly from a bank account).
Second, because convert-transmit-unconvert still bypasses many of the painful aspects of other money transfer mechanisms. Both buyer and seller only interact with entities they trust (coinbase and their bank), no credit cards (and associated fees, chargebacks, and headaches) get involved, and the transmission process in the middle uses bitcoin in lieu of ACH or similar (avoiding many potential risks, reversals, and the various Bad Things that can usually happen if you pay directly from a bank account).