I really don't understand why people don't get this. If you have $100 to lend and want a return on your investment and you had to charge everyone the same interest rate, would you lend it to the person with better credit or worse? Obviously better credit. So if you want to lend to the person with worse credit you are going to charge them more because of their history they will default more. The risk is priced in to the interest rate.
Because credit isn't an indicator of future repayment (income is more so). The amount of people who defaulted on their home mortgages when they were underwater after the housing collapse who also had good credit prior is not insignificant.
Anyway, they defaulted because they were underwater on the mortgage. When you owe 50% more than your home is even worth, bankruptcy looks like a viable option for many people. Same reason lots of people sold their 401k/IRA/investments. Fear has a fun way of becoming self-fulfilling.
Credit rating is explicitly intended to be an indicator of future repayment. That's the whole point of the system.
The more likely you are to pay back a loan, the better your interest rate, as the creditor is assured they will make a complete return on their investment.
When the chance of you disappearing or declaring bankruptcy is high, you are considered high risk and so your interest rate is increased to make up for that risk. At least that is how I understand lending.