Its a hell of an undertaking. The scale is huge for the track itself, but also trackside objects etc.
And how many solid track makers are there? Could probably count them on one hand, and slowly cross them off when you consider their preferences (US, vintage etc.) So yeah, I don't find it all that surprising its not been done, not when the vast majority of users are happy to download mediocre converts over and over again...
I would love to see it happen though. Maybe some sort of group effort, sharing resources between anyone who can contribute.