Second to last time I called Comcast I was upset because they lied about a service and its pricing in online chat support. Being quite polite I was hung on up 3 or 4 times, blind transferred repeatedly, had a few reps pretend they were supervisors, and when I finally got a callback from a real superior they demanded I provide the chat transcript or it never happened. When I actually had saved the transcript and provided it the supervisor said they weren't authorized to promise me that and would not amend the bill even historically. This process took three days of calling.
My last experience with Comcast was a business internet account. I called and canceled the service because I was moving, the rep was very helpful and took the date. They didn't ask for the modem back and in the hassle of moving I forgot completely. Several months later I received a bill from comcast at my new address. I thought this must be the final bill, it included a charge for the cost of the modem. I paid it figuring that would settle everything, the rep I spoke to when I called indicated that they had no record of me canceling which is why the charges were higher than expected but that she'd corrected it and this payment would settle everything. Three months later I get another bill which includes a charge for the cost of the modem again. I call and they have no record of me canceling, can't explain why they are charging me for a modem if they don't think I've canceled and or why they are trying to charge it again but the charges have been corrected and the system is updated. A month later another bill, a new charge for the modem, call again and they indicate they are charging me for the modem because it wasn't returned. When I point out that I've now moved and there is no way I could possibly return the modem they indicate they will continue to charge me the full cost of modem every month until it is returned. Finally, I report to the BBB and their response is similar with an additional claim that it doesn't matter if I called and canceled or that their representative indicated the request was submitted, etc because somewhere in the fine print on the agreement it indicated I'd have to cancel in writing. The BBB cleared them automatically because I eventually gave up on going round and round, Comcast can afford to pay drones to auto-reply longer than I can and the BBB is a joke.
After the BBB request they stopped trying to add charges and a couple years later I successfully booted them from my credit report. Your millage may vary but more likely you've just been beaten down by lousy customer service so hard that you just think this is how it works. Most companies have reps that either say they will do something to resolve issues or won't agree to do something... every so often I have a fluke where something gets messed up but Comcast is only company I've interacted with they consistently lie saying they are resolving the issue and then not.
Yeah, AT&T sucks too if anything goes wrong but in three years of my wife having cell service with them I've never had to call. With comcast it was every couple months.