Why do cargo aircraft still have floors? Even with dedicated cargo aircraft built as cargo aircraft (rather than converted from a passenger-carrying configuration), the cabin is virtually always still divided into an upper and a lower lobe (which would, on a passenger aircraft, be the passenger cabin and the cargo hold, respectively) by a floor located a third to a half of the way from the bottom of the fuselage, drastically reducing the maximum size of the cargo that can be carried and making it much harder to transport round outsize cargo (such as, for instance, large high-bypass turbofan engines as are used on essentially all modern jetliners) by air.  Given that the essential structural members of a large modern jet are


the skin itself;
longitudinal longerons running the length of the fuselage just beneath the skin; and
circumferential frames encircling the fuselage just beneath the skin;


it should be possible to remove the floor, and thus dramatically increase the freighter's ability to carry very large objects, without significantly compromising the fuselage's structural integrity (the middle of the cabin would still be partially occluded at the bottom - or the top, or the middle, depending - by the center portion of the wingbox, but there would, nevertheless, be a considerable increase in the outsize-cargo capacity ahead of and behind the wings).  Why isn't this done?  Is the floor left in to facilitate possible future conversion to a passenger aircraft?
Because the overwhelming amount of cargo is fairly small and floors allow easy loading of both decks at the same time which is more important than the ability to load big cargo on occasion. By having floors you can load standard cargo containers quickly and efficiently. It also gives you the ability to standardize the containers with a reasonable degree of reuse most importantly they can fit into the lower deck of a passenger aircraft with out modification. Passenger airlines make a lot of money moving commercial cargo.   

When it comes to flying around big, cumbersome cargo there are simply aircraft designed and built for such a task like the Airbus Beluga



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