Because it's convenient?

My parents use their Platinum cards whenever possible, but they never redeem the membership points or use any of the perks (airline reimbursement, lounge benefits, concierge, purchase protection, etc.); for them it's about having a reliable charge card they use and pay off every month. For anyone that doesn't accept Amex, they have a MC/Visa debit card. Heck my parents don't even use online account management. They prefer writing a paper check every single month.

Personally I use every benefit that is offered whenever I can (except concierge).