The reason is privacy. It's not going to spread the loss of a hack because if the hacker gets the wallet they own all the addresses. One of the core developers

The reason is privacy. It's not going to spread the loss of a hack because if the hacker gets the wallet they own all the addresses. One of the core developers theorized that memory corruption at the exact time you sign a transaction could lock away all the bitcoins in a single address. However this has never been observed. (remember every transaction spends the entire balance of an address and returns your coins to the same/or change address, if that change address has 1 bit wrong the entire balance would be lost)