I'll take a stab at this one:

opening compartments require more space. Any place that has a compartment needs to have that area waterproofed off of the rest of the device. When it's a thing that can be removed, it may not provide as much structural rigidity, so it can't be trusted in as part of the foundation of the device.

If the same form factor was to be maintained, then, since you need a compartment to store the battery, that takes up extra space that can't be used for more battery, so battery life would suffer, too.