Why does bitcoin create several "empty" wallet addresses?
Bitcoin doesn't "register" addresses on the blockchain. The reason I was having trouble looking up arbitrarily-generated public addresses on the testnet Block Explorer website is because I was searching for mainnet addresses, not testnet addresses. For example, addresses generated using the above https://bitaddress.org website are by default meant for mainnet, and are invalid on the testnet blockchain. If you use add ?testnet=true to the URL you will generate valid testnet addresses.

The exact differences between testnet and mainnet addresses are documented in numerous places online. Among them, the Testnet page on the Bitcoin wiki, which states:


  A different value of ADDRESSVERSION field ensures no testnet Bitcoin addresses will work on the production network. (0x6F rather than 0x00)