It may be fairly safe, but it's not risk-free (and certainly not guaranteed by the government). Even if companies don't default, their borrowing rates can still

It may be fairly safe, but it's not risk-free (and certainly not guaranteed by the government). Even if companies don't default, their borrowing rates can still increase, reducing the value of the bond and causing you to lose money. The 3% return is compensation for the added risk. The financial markets are pretty efficient for liquid stuff like this. Banks also provide services that consumers are willing pay for through lower rates on their checking accounts, and need to cover their administrative costs or have some other way of making money with the deposits.