Because there’s a whole economy to designs and vinyls, and apparently it’s literally impossible to figure that you could give credit and credits to all the creators of vinyls included in a livery, or that creators could opt-in/opt-out of allowing/forbidding people sharing liveries with their vinyls.
Then again maybe this is a lot to ask for a game that knows you’re trying to import a livery with a Horizon Festival decal, and won’t give you the option to import it without the decal.
Edit: You might be interested in these suggestions
Setting to share Vinyl Groups as “Open Source”
Allow sharing liveries with locked decals