making a mod with copyrighted material is illegal. Whether the holder of the copyright chooses to pursue the matter is all the keeps most mods available. You can make all the cars you want, but the moment you slap a skin without permission with real sponsors and logos you become a contract violator. Blancpain pays SRO alot of cash to present their name on the cars and the GT3 series. If they wanted to, they could demand SRO issue cease and desist letters to every mod creator that features Blancpain GT3 liveries. (The beauty of this is Blancpain doesn't have to do anything, just inform SRO that SRO MUST police any content containing Blancpain. So SRO has to pay their lawyers or risk Blanpain recinding the agreements and demanding refunds.) A C&D letter is NOT a law, but a threat, comply with the letter or the SRO lawyer team will begin legal proceedings that will cost buckets of cash to defend. They don't have to prove a dang thing, just force the mod creator to be financially devastated.
When iRacing first started, almost the very first thing they did was revoke permission to mod the last Sierra Nascar title into the Group C mod. Sierra had given the group permission, but when iRacing bought their intellectual properties back from Sierra, they terminated the agreement. The modders fought it and did get a partial victory, but if at least one of the members wasn't financially able to resist, they would have been forced to pay their own lawyers and probably the iRacing lawyers as well. When Papyrus/Sierra was first expanding their Nascar titles, one of the biggest tracks NOT available was the Daytona 500 track. Modders even created a similar shaped track named after the county(Volusia) But when the copyright holder discovered those efforts, they sent the letters and forced the track to be removed from any site it was discovered on.
Technically, ANY sponsor displayed on a car, for which they paid money to display a copyrighted logo, could force any mod containing the logo to be removed. All they have to do is have the will, or ill-will to proceed.
So NO I"M NOT JOKING. This isn't the big bad government, it's legally binding agreements between parties that pay for exposure or exclusivity or whatever. The agreements almost always include PROTECTIONS from the money being that is paid by one party to the other not to be wasted by unwanted competition or downright theft of copyrights.
At this point in time, few companies seem willing to chase these mods, but that could change in a moment.
This is why you see companies like S397 & RaceRoom. being so careful to license their GT3 & other content prior to releasing anything. And finally, even if the mod is free to the public, that is not a defense. The very existence of a free mod could be argued as diluting the value of the legally licensed products. It is why websites with corporate sponsors, like the late No-Grip rarely had copyrighted material available for download.
You may have noticed S397 doesn't have a download section, they are relying on sites like Race Dept to make the possibly illegal content available rather than on their own site.