What good is it to tell insurance companies they can't exclude people with preexisting conditions?

The insurance companies can then just charge as much as they want to people that have expensive preexisting medical conditions. For example you have diabetes, you go to get insurance, the company says sure we would love to insure you for 1000/month. Who can afford that? The government would have to also tell the insurance companies how much they are allowed to charge for it to help any.