The problem is that there isn't a lot of wiggle room in the USB-C spec to make a compliant plug thats attached magnetically rather than mechanically, and the contacts are recessed rather than surface contacts (so it has to be 'plugged in' rather than 'resting on')

Griffin sells a magnetic USB-C cable - that winds up being a magnetic dongle to a charging cable. Apple doing this would have been considered a hack against their design and would have required them to include a lower quality data-only cable rather than the data+charging USB-C cables they currently include.