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.
It wasn't cheaper than all alternatives. There were a bunch of virtual operators offering better monthly rates than the big networks but I've personally had bad experiences with network deprioritization on them. Depends very much on your individual circumstance, I'm in NYC and the network is clearly pretty saturated.
If this isn't obviously the case, the "normal" plans are subsidizing phone sales. This means that you're almost certain to be better off with a vendor that doesn't offer this or at least offers it in a way that isn't dishonest.
At the end of the day, even if what you say is true making use of this deal makes things worse for everyone because it is part of a larger strategy. Inevitably you will be squeezed for more money than you otherwise would, sometimes it just takes a while.