The economic incentives are different. AT&T was the monopoly phone company subject to rate-of-return regulation. The more they spent on CAPEX the more they could raise prices. So naturally they spared no expense in maximizing the reliability (99.999% uptime) and quality of their network.

Then cellphones and the Internet happened (as much as they fought it). Why invest in a dying industry? Now phones are used to send picture messages and get addicted to watching one short form video after another.

People value mobility and having one phone bill over toll quality voice. People get much more annoyed at "All circuits are busy. Please try again later." errors or 5 cents per minute than mushy call quality. Making phone calls from a car or walking on the street is always going to be noisier than at home.