They know if the game is open to modding, the cash flow they are getting through Patreon will very quicky fall flat. The dev wants to keep the game firmly in his palm and control any content for it. A few modders could concievably actually have this game finished in a matter of months and introduce a cornicopia of new variants through it's very well built character creator (Which is probably the most valuble asset for the game). Hell even I could write a more engaging breeding system than simple RNG. All he's done this year is add a new character, some new sex animations and the bare concept of clothing shop. (Although I don't know if the clothing will add anything to the game past pure asthetics.) No new variants, no new quests, no new areas to explore, not even an endgame goal. Hell even throwing in hidden items to discover and collect would be better than the "Fuck all" we get every half a year or so.
There's been too many breeding games that have fallen victim to a failed development or an endless cycle of life support, hence the term "The Breeding Season Curse".
I put my support to Porn Empire. It's engaging, addictive, open to mods and the dev actually gives a shit. He has never released a disappointing update.
https://f95zone.to/threads/porn-empire-v0-94-dev3-pedev.2857/