I have a feeling though, like others have mentioned, the arms are designed that way.
Like a crumple zone in a car that is designed to be the flex, crumple or in rc arms break area.

If the arm was designed to resist failure more so than the bulkhead, you'd be tearing further into your car to repair it. If your hitting a post or whatever immovable object at a high rate of speed, something has to give. I would prefer to change an arm instead of something further down the line.
Think of the screw count to get to the lower bulkhead compared to changing out an arm.