Not every web service can mail you a physical symmetric cipher, banks maybe, but they focus on making online banking much like PayPal than providing the hardest security they can implement. On the other hand, the SSL/TLS protocol does just what described (symmetric key delivery) in a pretty automatic way, by providing the browser a public key, the browser generates an encryption key locally, encrypts it with the server's public key, then sends it to the server, which will use it for future communications with the client.

At $20 they are still expensive compared to the total production cost, lasers, motors and circuit board components are pretty cheap at today's wholesale prices, and compared to the cost, speed and benefit of Flash Memory and SD Card, we can clearly see that CD/DVD technology should get out of the market, that $20 can get you a blazing fast 32 GB flash drive, which is also solid state, stable, durable and very portable.