What's the reason?
Well, he's misunderstood you slightly, but the reason would be that it would massively increase file sizes if all bricks saved all variables.
Of course, none of the default variables actually need saving, and unless some moron went all-out overboard with variable tagging, it still wouldn't increase file size too greatly.
It would also be theoretically possible to set the name of a brick to have a 'special' value stored in it. Unfortunately, any way to do this that doesn't break some element of the game also allows users to do this manually, so it gets rather complicated as you have to start checking to make sure THEY'RE not setting 'special' values.