I have an example of the opposite, OppaiMan, their games seem horrible to me, the crudest expression of a porn game, but I admit that they knew how to take advantage of the money they earned with the first updates, they went from being 2 people to a studio that plans to become the Steam of porn games. In 99.9% of the cases the excuse for not increasing the development team is "only I, the original developer, has the talent, vision, and love, to create this VN, I have tried to hire people but none of them are at my level."
I already accepted that the update will come out in July/August, during the summer for most people here, that's how the industry works currently, even games like Poppy Playtime receive one new chapter per year, although I would propose that people who are currently in summer, like me, receive the update with 1800 renders.
*I will send you the update via private message, don't worry friends, just don't make noise*
I've seen at least a few cases were the original dev has tried to hire help, and off the top of my head, over half of the hired help flakes and vanishes within a few months. If I had decent-level skills, would I take actual studio pay or get by on a meager slice of the money many creators on here are collecting. The typical income in the US for someone with actual graphic artist skills exceeds what many AVN devs are pulling in from working on these games.
Second, as you note yourself, OppaiMan games aren't great. How much of that is because they just hired whatever people not putting in the greatest work. With many of the AVNs I've gotten, they are something of a passion project, and hiring any random half-assed person would seriously downgrade the quality of the game to OppaiMan or worse. I went thru my collection, and I only have one that is made by OppaiMan themselves, and one or two others written by others but available on the OppaiMan store.
Third, it turns out making these games isn't cheap. For many of the solo dev games I follow, the first bottleneck to overcome isn't having a second person working on it, but the length of time rendering takes.
There are some cloud-hosted rent-by-the hour rendering solutions. I priced one out a while ago based on one dev's detailed account of a delay due to having to rework animations for a single scene. The release had the equivalent of 45-60 days of 24/7 rendering on a dedicated 3090, plus using a second machine for writing, posing, and any other non-rendering work. Found an online calculator for one of the cloud-based services, and it would have ranged from $12k-$15k to do the rendering using their service, albeit it would have been much faster. That dev gets about $5k a month from Patreon. They weren't US-based, so maybe they can survive with a roof over their head on that kind of revenue after all the expenditures that come from making a game, but it doesn't sound great.
If I were to suddenly decide I wanted to make it as an AVN dev, maybe I could get away with just sinking thousands of dollars into finding an unobtainium 5090, but to increase render capacity to anything close to a healthy release cadence and be in need of a second person, I'd want a dual RTX 6000 workstation (just priced one out at $22k) for rendering and a second pc for writing, posing, and compiling. A second, full-time writer/dev or maybe just a dedicated scene composer, I'm not sure two of the dual RTX 6000 machines would keep up for a higher image quality game.
So, configuring a pair of pcs to do the work, buying all the assets, giving up a day job to commit to it full time, I'd need minimum $75k in spare change laying around for startup and to hold over until I got a following, with $20k or more in monthly income within the first few months to produce releases at speed with above-average quality. That amount wouldn't even pay back the startup costs over the first year or two, and wouldn't cover the salary of a second person working on it.
Somehow OppaiMan is drawing almost top-15 AVN numbers on Patreon, while also running a marketplace to sell other's games. I guess they have found a niche people are willing to spend money on, so good for them.