Most of them are available, here:
https://assettorallygrb.wixsite.com/rallygrb
Only some fantasy R5 cars missing, and the Delta HF 4WD.
I'm understand what you want to say, but not totally agreed with you.
- On that YT chanel only some cars converted from RBR, the other half of them come from Dirt.
- A crappy conversion can be updated, see the F1 rework projects here! (1993, 1996)
- If somebody a fan of a car, for example the Fabia WRC, he would like to drive it (or at least try it). And don't want to drive a Subaru instead of Fabia.
- I don't think that 'scam request' is to find a car seen in a video, which is not exactly describe, that the car is part of a "private mod".
Finally,
I tried a lot of rally cars in AssettoCorsa. Maybe in Tarmac there is a good level of the physics, but on snow, and gravel I think RBR is still the best today. The weakest part of RBR was the sound, but some weeks ago, FMOD is integrated to RBR opening a new future of the 16 year old game.
I tried DirtRally 2.0 too. It is better than previous version, the physics is really good. But even with all DLC's, it haven't got so much and various tracks and cars as in RBR.
So I think Rally, and Dakar... is not so suitable for Assetto platform. It is good for Hillclimbing and Circuit Racing, Slaloms.