The issue being why is there not more ________ fill in with favorite railroad routes or content?
I think the most important difference between between the Trainz world and the Model Railroading world is that there is no Athearn or Atlas. All "models" that we use were scrap-built by another hobbyist. Once a prototype model is scrap built there are no limitations in producing and distributing it, nor on how it is painted or weathered. There is no kitbashing, however. You either like someone elses model (and perhaps weather it, or change the road numbers) or you need to create your own, from scratch.
Because of this I find it very hard to compare it to a model railroader community. I liken it more to a video game modding community (since essentially that's what it is.)