One has to guess, that the bees produce the hex structures they do because its an evolved, stable behavior. A more complex innate behavior might be fragile (change abruptly with a small change in genetics) and produce disastrously non-adaptive honeycomb. Any foray into genes toward producing rhomboid (or whatever) structures may have intervening steps that produce leaky, inefficient or structurally unsound layers and doom the hive (and thus the modified genes) before it got 'all the way' to a new, arguably more-efficient hive.
In other words, they are in a genetic 'relative maximum' and any change would be to a less-stable and less-efficient place evolution-wise.
I would argue that we, humans, as mathematicians, carpenters and apiarists have worked together with the bees resulting in a net gain in honey over time.
In other words, they are in a genetic 'relative maximum' and any change would be to a less-stable and less-efficient place evolution-wise.