The floor of course could be arranged to be horizontal even with a +ve pitch on the hull, however that is apparently not done:-)
I suppose as most aircraft fuselages have a circular cross-section, the installation of a sloping floor would involve a tapering planform, which would add to complexity and cost.
The crew bunks on the 747-400 were slightly inclined, down towards the nose, which made them flat. Very comfy they were, too!