Even if devices weren't ultrathin they'd need to be in protective cases. Cases can be damaged and easily replaced. A damaged device is more expensive and less convenient to repair or replace.

As for battery packs, I have multiple devices, but I carry one battery pack that can charge all of them as needed. My phone is much lighter in my pocket for not having to have a larger battery (that I don't usually need), my ipad is easier to handle, etc.