If the manufactures made upgrades and released then to all the phones that were unlocked at carriers that would allow a upgrade.
There is an upgrade. Go to Kit Kat.
Then the carriers that would not allow upgrades would get angry customers. And a chance to loss them if they had a choice to pick someone that would allow upgrades/updates.
I don't think you really understand the mobile market in the USA. One of the carriers fucks the customer in a new and interesting way, the others follow. Switching mobile carriers is 1) a huge inconvenience and 2) not going to improve your situation, because they're pretty much all equally awful. The carriers don't want to support more than one version of Android on a given handset, because then they might have to spend money to support them. Obviously they can't do that, since it eats into the profits. And as long as nobody goes off the reservation, the situation will not improve. Yes, collusion is illegal. Their lawyers can beat up your lawyers.