If consumers paid out of pocket for their phones then they would be more picky about upgrading and plan prices. It would also make upselling shitty plan features harder so the carriers would loose a lot of money.
There may be a really good reason for a not well-off family to get a new and advanced phone from the phone company, for a small monthly payment. They can't afford the upfront cost, and will pay more for a depreciating asset. On the other hand, they now may have a phone with a great camera to record their kid's school graduation, or other such event that only occurs once. Or they may finally use a smartphone with 4G / 5G to have good-quality video calls with some faraway friends or kin, which were a pain with their old phone. Etc.
This still beats buying a new phone with a credit card, at 29.95% APR.