Can anyone tell me why I can't...

On one hand I have an iPad Air 16GB in great shape only issue is device is activation locked and I gave up years ago fighting with apple about getting it unlocked when I could prove the previous owner was my great aunt who is dead. On the other I have an iPad Air 128GB that has needed screen replacement 3 times, and battery replacement twice. The current battery in the 128GB is swelling beyond belief and has begun pushing the display off.

Tell me why I can't just swap the board from the 128GB into the body of the 16GB.