Hydraulic disk brakes usually adjust for pad wear via special seals that allow the pad to "creep" closer to the brake disk as they wear down. It seems that such seals could equally well be used with mechanical disk brakes. However, all mech disks I've seen require manual pad adjustment. Why is this so?
I think the difference between hydraulic and mechanical disks is that there is no seal on mechanical calipers. You are saying that the seal "rollback/slip" mechanism doesn't require fluid to operate, but it's important to note that mechanical brakes don't have a seal. If you consider the brake pictured below you'll see that it's an open system requiring no seals and just has an actuator pressing against the pad, with a spring to retract it.
I'm also not sure what the added benefit would be, since the usable thickness of the pad material is only a few millimeters, requiring only a very small amount of adjustment over the lifetime of the pad. You could easily just adjust when needed as it's not likely that you would need to have the pad be adjusted during a ride.
Also worth noting that in order for the rollback and slip to work, it has to have a sealed chamber behind the pad, and even if it was sealed, it would probably need to be filled with hydraulic fluid, since if it was only filled with air, the compressibility of the air wouldn't have sufficient force to pull the piston back from the seal.
In mechanical brakes, the cable is of constant length. It can't become shorter or longer.
In hydraulic brakes, you have a reservoir of hydraulic fluid. If the pad creeps on the seal (after the seal has "rolled" already as much as it can "roll"), the lever will move closer to bars the first time. Then when you release the lever, reconnecting the reservoir to the active working fluid in the brake line, the new empty space in the line will be filled with new fluid. The next time, the lever won't move closer to the bars anymore as this time the pad motion is only made by the seal "rolling" and not by the pad creeping on the seal.
A mechanical brake would need something equivalent: a device that would offer new length of brake cable, just like the hydraulic reservoir offers new working fluid.
I'm afraid such a device can't be easily made in small format, to be small enough to fit inside the brake lever. This is not a Swiss watch with minuscule forces, but rather a device that has to be capable of transmitting great forces.
Mechanical-hydraulic hybrids that use a mechanical lever, a mechanical cable, and the brake master cylinder is near the caliper, and the cable just actuates the piston in the master cylinder, could obviously have such an auto-adjust feature. But purely mechanical ones, not so easily.