This is primarily a text-based game.
The bald, featureless silhouette is a concession. A one-size-fits-all silhouette that leaves details up to the imagination means only one silhouette is needed per pose per species. If more details are absolutely necessary for the scene, they've already been delivered in text form.
Beasts, beastboys/beastgirls, tentacles, swarms, etc. require (and have) different silhouettes/images because representing them with a plain human silhouette would be more distracting than useful.
Since one silhouette per pose per species suffices, dev time can then be funneled towards new areas, scenarios, named NPCs, and mechanics instead. It is these things that are the lifeblood of a sandboxy text-heavy game.
If you want to redraw silhouettes to show proper details, then you'd need a mountain of variations to fit the possible appearances/situations.
As it stands, using your redrawn attachment would mean every human partner has "loosely-styled long black hair, pale skin, large breasts, and no clothing" when riding the player character.
The more art variations you add, the higher the expectations rise to cover other appearances/situations accurately; it's a hungry beast once you acknowledge it's there and start feeding it.
And once art variations are done, since the bar for representing scenes visually has now been raised, attention instead shifts to why minor pose changes are not represented accurately (eg. fingering, sucking breasts while performing another sex act, support for over 3 silhouettes surrounding the player during gangbangs, and so on and so forth).
It's a slippery slope, so drawing a clear line early by sticking to featureless silhouettes makes sense, especially since it's not a graphics-heavy game to begin with.