A much better question is why pare some back? Closing credit cards that don't have an annual fee has zero benefits.
That said, I'll answer your question: One component of your credit score is AAoA (average age of accounts) and it takes a hit when you close an account and it gets deleted from your credit history after a couple of years. So if you wanna minmax an 850 and get very favorable terms (well below inflation) on every loan (cards, cars, mortgages, etc), having very old accounts with a small/zero balance is optimal.
(For the same reason, closing recently opened accounts can improve your score after they get deleted from the credit report a couple of years after you close them)
That said, I'll answer your question: One component of your credit score is AAoA (average age of accounts) and it takes a hit when you close an account and it gets deleted from your credit history after a couple of years. So if you wanna minmax an 850 and get very favorable terms (well below inflation) on every loan (cards, cars, mortgages, etc), having very old accounts with a small/zero balance is optimal.
(For the same reason, closing recently opened accounts can improve your score after they get deleted from the credit report a couple of years after you close them)