Look at the "Installation" section in the first post
It was mentionned plenty of time ^^ But that's just how the game is made.
When you try to scout all of the internet for representation of all the game lessons, SFW and sexual... It's already hair pulling how little usable art there's available. Before working on the game, I'd picture hentai pictures of footjob for a milf with short brown hair would be legions, well no. So imagine for all the variety of art and slaves? There's currently thousands pictures in the game... For sure there wouldn't be enough available pictures one virtual character.
As for patreons, it would need an absurd amount of money and time to make it "full commisions", and when I tried to make one Patreon banned it anyway (bestiality, beating and rape were mentionned in their banning message).
A guy tried to make a mod that would put manga characters have their own scenes with only them. I even offered to help. He seemed very motivated. But we got no news from him and that was a year ago.
If one day a guy wants to make a mod with unique slaves, I'll help him gather quality art. But I wouldn't have too much hope.
Besides... The game in its current state has nice art. Some people get butthurt over it being "stolen" on a piracy friendly site
But really, it's a good portfolio of uncensored hentai and the criterias are specific enough so that the sex looks like your slave, depending on your suspension of disbelief. I wouldn't change it for the system of Slave Maker and Brothel King, which has art tied to every slave... But less sexual options and more inequal art quality.
But yeah, if I had a wish granted by a magic genie, I'd want the game to have unique commissioned art, it sure would be nice. But not a necessary condition to make the game visuals good in my opinion. Speaking of that, this department has been HUGELY improved since the original old huntsman version; several thousand of non-generic pictures, censorship removed, no weird gif loops with x-ray... So we've already brought the game's original concept to the best version of itself, it's something to enjoy