I'm pretty sure he was asking because there is current flowing through that wire.....And if there ended up being too much curent passing through the wire, it could melt the jacket and destroy the wire. Especially in a long neg. run like from the trunk to the underhood batt. Not really a stupid question if you ask me.
The answer is because if there is too much current, the + fuses will stop teh flow of current, and because of this, the flow of current in the - run will be stopped. Because of this, as long as you fuse your + run, there is no need to have a fuse to protect your - run, since it is already protected by the + run fuse.
Maybe contribute information to a thread instead of just insinuating that people are stupid and it should be done the way you say because you are einstein