Different idea for cargo transprts

I was wondering why cargo transports all have to carry their cargo internally.  Why aren't large cargos handled like barges or tractor trailers.  The cargo would be put in a cargo container which would be lifted off world some kind of atmospheric transport.  Then they would be lined up and tied together for a cargo engine to push to the destination.  The ship itself would be just a small crew compartment, and some big mama jama engines.  The hauling capacity would only be limited by the size of the engines and the weight of the cargo in the containers.<br /><br />Now a ship like this would be very vulnerable to pirates.  It would not be very maneuverable at all, and it would be only very lightly armed because of the small size of the ship itself.  So, to haul through dangerous space, it would need some kind of an escort.  Though it might make for a good trap to have a couple of the cargo containers loaded with rocket or laser batteries.  <img border="0" src="/modules/tinymce/tinymce/jscripts/tiny_mce/plugins/emotions/images/smiley-yell.gif" alt="Yell" title="Yell" /><br /><br />