reestablish credit

My dad has at least eight credit cards in his wallet, with a $0 balance on about six of them. The balance on the other two is around $50 in total. But he tells me that the credit limit for all those cards combined equals around $15,000! So, why he doesn’t use them more often, when clearly he could use the money? His answer: “I don’t want to have to pay all that back.” And that is how credit cards should be used.