They don't do this because the trucks are outfitted with a big "robotic" grasper that literally picks the "can" up and dumps it into the truck. And sets it back down, ostensibly in the same place where it picked it up from. (at least here in So. Cal, and probably lots of other places.) Designing/building/implementing something like that that could handle as unpredictable of a load as bags would be prohibitively expensive.
But building a little motorized cart that could carry the can to the curb should be almost trivially easy - just have it follow a wire, or for that matter, a slot! If you get snow, then a snow-blower attachment could be an add-on. ;-) But seriously, this is sounding increasingly simple - a low-profile 4-wheel dolly, with a large wheelbase (for stability), and a reversible motor, like a wheelchair. The technology has been in place for decades, there just hasn't been the volume to get the price down to pennies yet.
And to keep anybody from stealing it, tether it. :-)
Good Luck! Rich