I think I need to explain what I know about the DTM licence and all other series.
When a dev buys a series licence such as DTM, it is for the legal right to display the name and logo of the series. The dev team then need to then buy the individual rights to the vehicles from the manufacturer, each track and liveries from each team.
The track restrictions come directly from the DTM licence holders; if R3E allowed the DTM experience package to race on non DTM tracks, S3 would contrivene the agreement specified by DTM management to run DTM cars on only DTM tracks via the DTM experience packages.
The lack of DTM series licence is why the DTM cars are now grouped by year and can be raced on all tracks in R3E.
If dtm said "okay guys, if you want the license, you come directly to us but we have these technical restraints" we would've seen dtm in other sims too since dtm still isn't dead and somewhat popular. But i still have hope. Sector3 didn't renew and after the convergence of supergt and dtm is complete, we might see a dtm series in one of our sims after all. I love me some silhouette racing.
I'm not sure I follow "our sims".
You can find DTM cars in project CARS and older DTM cars in AC, although neither has the official licence. There is nothing stopping any dev team from getting a DTM or any other cars such as GT500, V8 Supercar or BTCC directly from the manufacturer as it is their IP.
rF2 has a BTCC Honda and GT500, GT1 and GT4 Nissans amongst other licenced vehicles without the series licences.
Further, United Racing Design has a T5 mod for rF2 and Assetto Corsa so you can enjoy the DTM cars in these sims and there is probably all manner of rips for rFactor. From memory, DTM cars are also in Grid Autosport.