It's harder than pushing
a firmware. You need to write and validate a special firmware with a specific weakening of some measures, and then add a protection to lock it to that specific phone so it can't be used as a master-unlock and verify that. Because of how the FBI is pursuing this, Apple will have to take all the risk. If the update bricks the phone or fails to recover the data, Apple's reputation suffers.
As for storage lockers, like safe-deposit boxes, it's only reasonable for the government to ask the provider to open the locker/box because the business has the means for this. Non-payment results in eviction and probably "every day" they have a maintenance person sawing locks off. On the other extreme, Apple doesn't need to have a way to decrypt and evict your data for their general business so it's unreasonable to expect them to be able to provide this.
If the government wants this lock sawed off, it needs to do it itself. (Or pay a hardware-reverse engineering company do it for them.)