My issue is that my bar keeps going up. The bad new games are easily forgotten, and the good new ones just keep raising the average in my selective memory.
I Disagree to an extent, I think its because more exceptional games are coming out that the good games that you would once praise are being diluted by the pure quality a few hold over it.
it is no longer at the point where good renders and proper(ish) English are enough to be successful.
Will say it does not help that the growing trend of good titles being abandoned half way through once the dev decides to cash out with <insert medical issue here> excuse leaves many people hesitant and unwilling to invest into projects from unknown developers causing talent to invest less into the product as a result.
Man, I kind of agree with both points. The standard for newly released adult games keeps getting higher. Like, Frank's Adventure, SimGirls, and Meet N Fuck were classic series in the early 2000s, but hardly meet today's expectations in terms of art and animation. While I still love Frank's Adventure, it's easy to forget how crude some of the old games were.
That said, I do think there are a greater proportion of disappointing new releases today when compared to the adult games of the late 2010s up through about 2023. And you're definitely right about more devs cashing out and abandoning titles mid-development. So, what changed?
Well, as others have already pointed out, the prevalence of the same store-bought 3D assets and the rise of greasy-looking AI image generators have been, at best, a mixed blessing for creators. Great ero games can be made with those resources (for example, Treasure of Nadia with stock 3Dcg, Wasteland Lewdness with generative AI), but there's a lot more low-effort slop out there to sift through. IMO, the combination of asset markets and AI is why good renders alone aren't enough for a game to get an audience any more.
Additionally, internet content has become more commercialized and Google search has become demonstrably worse since around 2016. At least in the USA, legal challenges to net-neutrality mean that Google's algorithms no longer give content from smaller producers the same weight as content from large corporations, a situation worsened further by the rise of generative AI around 2021. The end result is that high quality amateur ero games increasingly struggle to get exposure in competitive markets. Blogs and social media are struggling (Twitter is moribund), and the commercial marketplaces like Steam, Itch, and Patreon are riddled with censorship, leaving specialized forums the best option for finding quality ero games.
There may also be other factors at play:
1.
Pandemic Boom and Bust. The pandemic gave amateur game devs a lot more spare time, and loads of people were stuck at home alone and horny. Hence 2020-2023 saw a boom in the production of adult games, followed by a dip and lots of abandoned projects as creators went back to work. I think this is the best explanation for the sleugh of abandoned games recently.
2.
Death of Flash Culture. 1996-2020 was the golden age of flash games, interactive fiction, and web development. Lots of amateur devs learned how to make games because web design was taught in school. The barrier to entry nowadays is much higher, as compared to Flash, Quest, RAGS, TADS, and other venerable engines, many of the modern professional-grade engines are a bit "heavier" than needed for an amateur project. Godot, Unreal, and Unity are powerful but hefty, with lots of auto-generated junk files--frustratingly inelegant and overkill for most amateur producers. (Godot is great, but requires more programming know-how than Flash if you really want to get the most out of the engine.)
The "death" of Flash also slowed down or killed off a lot of interesting projects and creators (RIP Peach's Untold Tale). HTML games have been slow to fill the void left by Flash, with some important exceptions (Free Cities, Degrees of Lewdity, and Incubus City, for instance). Some big projects like Fenoxo's Trials in Tainted Space did succeed in the transition from Flash to Java & HTML, but there was a long period of development hell before the new ports could be made.
Other classic creators simply seem to have aged out or decided to focus on non-adult projects. For example, Mittsies, well known for degenerate flash games in the 2010s, seems to have decided to focus purely on music, and found success with Helltaker.
3.
Patron-Based Models Create Perverse Incentives. The rise of the Patreon model and the opening of Steam and other markets to "early access" adult games has lead to cash grabs, censorship, and all sorts of other drama. (Anyone remember the Breeding Season debacle? Nuff said.) Patron-based models tempt creators to cater to superfans and produce a constant stream of "content" with gated tiers of access. Then enshittification ensues.
Many of the "simpler" game engines are unfairly blamed for the woes of the patronage model. Ren'PY and RPGMaker games have been around since the early 2000s. Some are great, but many are terrible. The difference is that more of them are commercial now. While there have always been shitty amateur RPGMaker games (and I hope there always will be!), more of them used to be free fan projects. The golden age of web design promoted an amateur-focused, DIY spirit that extended to adult games. In contrast, the pseuo-professionalism of the Patreon era has lead to a lot of bad behavior and a homogenization of content.
In my opinion, there ARE still plenty of interesting and unusual games being made (especially amateur text-based games and games focused on particular fetishes), but there's also a lot more stale, low-effort competition from semi-professional creators.
TLDR: Why are there so many "stale" adult games on the market now?
1. The global pandemic is over so amateur devs have less spare time, and fewer people are developing games.
2. Flash culture is dead, and with it some of the old spirit of creative DIY. Modern pro engines can be a bit overkill for amateur devs learning how to make games.
3. The Patreon model encourages semi-professional devs to go for "safe bets" and thus less original content.
Those are my two cents, anyway!

Your mileage may vary.