You will probably find as many photographs on t'internet of bees working horizontally (think of all those flat roof cutouts or colonies between joists under floorboards). Bees just start at the top and work down until they can't go any further. there are a few examples around of bees taking residence in hollow horizontal branches instead of the usual vertical tree trunk.
And I always wondered what I would find if I had moved the gravestone in the chapel wall whether the colony living there had a nice vertical or horizontal space to colonise - it was probably a haphazard mix of the two judging by the multiple entrances they used.
Pondering on traditional African log or bark hives, the beekeeper had the choice when suspending their hives in trees to have them either hanging vertically or horizontally so what made them decide (on the whole) to have them horizontally. Same thought as to the evidence of early top bar hives Eva Crane found out in Africa (woven body with sticks as top bars) which inspired the modern 'Kenyan' top bar hive, what made the innovative African would be beekeeper decide to weave a horizontal container not a vertical one?