just because mophie may be able to do it doesn't mean Apple's engineers absolutely couldn't do it. my theory is that if they are able to make it, they probably won't support Apple Watch and would probably use less coils which would have less coverage (may result in lower charging efficiency == slower). essentially Apple engineers likely had far more constraints than mophie.
Why don't they just make a base with a few coils that sense where the devices' charging coils are located and move with servos to center the charging coil under the devices' charging coils rather than trying to layer dozens of immovable coils that tend to overheat. All of this comes down to cost of device including the R&D costs and manufacturing to market said device at a dollar amount that a consumer will purchase. Other words, with Anker and others making cheap single-coil charging pads and stands that "works" why would anyone want to pay so much for an "AirPower-like" device. "You" can make it but will "they" buy it? That's the question. I've tried the expensive multi-device Qi charging pads from multiple vendors and they are just too finicky for placement and the cheap Anker pads just work without bright lights etc.
how do you "sense" where the device coils are? need to figure that out while keeping in line with the QI charging standard so that all QI devices work, meaning you can't stick a sensor in the iPhone and have the iPhone tell AirPower "I'm at coordinates 32.1239, -40.9292 with respect to the mat".
then you have to figure out how to build the motor to move the QI coil in 2 dimensions in this thin device.
then you have to build three QI coils and have the motor(s) move all three coils (make sure you have enough power from a 30W charger to power all three coils + motor to move all three coils).
then what happens if you placed device 1 on the left, device 2 on the slight-left-middle, and device 3 in the middle, REMOVED device 2, then add device 4 on the right? does the coil in the middle jump over the coil that's charging device 3? seeing as how the arms moving the coil can't collide, coil #2 can't "go around" coil #3. the only option is for coil #3 to disconnect from device 3, move to 4, then have coil #2 charge device #4. but then what if you placed devices in vertical arrangment (airpods on bottom, watch on top)? can't charge like that. so then you have to think about how to tell the user "NOPE, can't charge like that, please move it to horizontal arrangement".
i assume the motor would add more cost, even more than adding 30 QI coils into a single device. and anything with a motor will die out quickly so need to test the longevity. and i assume the motor would add thickness to the device. and maybe the motor would likely be loud. are all of these compromises worth it?