I'm not the most advanced CG artist in the world, and I'm not certain what formats you're using to compile the environments, but honestly making TOP QUALITY levels really shouldn't be all that difficult. I'm surprised you're having so much difficulty finding someone to do it for you. Disregard the $$$. What formats are you looking for?
Jak isn't here anymore and I'm the main developer on the project now, so I can answer from my own perspective.
Making environments for this game and for my other game requires certain ways of doing things that are sometimes difficult to explain to artists for hire. Objects such as couches and tables and so forth all have to be set in specific places, at very specific dimensions, etc. in order to work properly with the animations.
The environments are generally made in 3DS Max and exported as FBX files which are then brought into Unity. Once the environment is inside Unity it then needs shaders and extra textures created for it, things like that. That's not something that can be done outside of Unity so it has to be done in the engine.
In the past when we had other artists do it, when it came time to put the levels into the game, aside from the shaders, etc. we also frequently had to move things around, sometimes slightly, sometimes not so much, to make it all work properly. That usually means destroying any lightmaps that have been created, as they have to then be re-made.
Add to that the cost required to hire out a 3D artist who can not only do kick ass environment work but who can also work within the dimensions constraints the games need, and the cost starts to add up. A high quality level done to the specifications required could easily run into the thousands of dollars. These games do not make a huge amount of money and are largely labors of love, so any added costs like that need to be weighed against how many units will sell of that particular add-on.
Basically, the process of making environments for these games requires someone who's very familiar with how the games and animations work, how it all fits together, etc. and can work directly inside Unity with the animations and figures. It's largely a back-and-forth approach, meaning the levels get finalized in the engine editor while testing with the animations. That's not something that can be hired out for very easily as it pretty much requires someone to be here in the "studio" working on it.
Plus, of course, some people just don't want to work on adult related game projects, so that cuts out a few more artists from the possibility list.
So, in the end, it is usually my partner who makes the levels (and she doesn't work on these full time) and me who finishes them up. We both know how it all works as a whole so that method works.
I'm not saying that no outside artist will ever make an environment for these games, I'm just answering the idea that it "shouldn't be hard" to find someone to do it.
With all of that said, I'm currently working on a new level for Sex Sim that will be released very soon.