Marketing controls everything, so there's no money or incentive to clean up or address the technological mess under the hood. Walk into any store and ask to have a prepaid and postpaid line on one account - they can't do it. Two separate systems.
They cannot keep track of phone lines, equipment bound to lines - all manner of basic, customer-focused use cases which don't bring in direct revenue, thus have zero priority when it comes to spending time and energy to get right.