For example, the call center you reach when you call the number on the back of your credit card is not the same one that calls you when the bank flags a suspicious transaction on the card. The first is an inbound call center hired and trained on service scripts, the second is an outbound call center that only handles the fraud checks and is probably serving multiple banks.
They're possibly not even in the same country. They're possibly not even the same depending on what time you call -- Comcast, for example, has inbound call centers in the US open during business hours, but routes calls to the same number to Indian call centers during late night hours.
Each call center has different training and different access to the customer accounts. The inbound tech support call center doesn't have access to the company's billing systems, while the outbound fraud verification call center doesn't have access to support tickets.
Having everyone a company tries to reach by phone call back at the company's phone number would lead to a phone menu with more options than buttons on your phone.