The parts alone are probably not enough. A lot of repairs require expensive tools (there was a video circling the net of an iPhone display calibration device -- if you want a proper repair, you need to calibrate the display after replacing it)

Also, increasingly there's authentication chips on parts like batteries and you need to somehow match them to make your phone not complain about 3rd party parts. So I assume you need some way to authorize each repair...

They may not benefit much from bothering to do so. It's unlikely to be high volume.