A lot of devs don't know how to alter the character meshes to come up with unique versions for their games. There are controls for just about every aspect of the model, later Daz models offer more and different ones or higher mesh counts. Another factor is the rendering hardware, as the higher-count models consume more resources so it may be that you can only have a couple of characters maximum in a scene since all your memory in the graphics buffers is consumed. And higher quality models take longer to render, plus if you include lighting and atmospheric effects that also adds to calculation overhead, making the process take longer. So if you are trying to turn a game around quickly taking the default models, minimizing the lighting and atmospherics, not doing as much post-processing, etc. means you have a lower-quality image.