I also had the displeasure of working for Comcast in phone support before I went back to school and this is definitely part of the reason.
Multiple completely different decrepit and kafkaesque billing systems because they were too lazy or incompetent to properly migrate customers over to a unified modern one. IMO, one of the reasons Comcast has had notoriously bad customer support is because their internal systems were so complicated that even reps who wanted to help might have just given up and told you they fixed the issue to get you off the phone.