A long time ago, I read one of their developers say you require the original games since the mod is specifically designed not to re-provide you with any copyright material from either game, and in fact I think it’ll look for and use the files you have like any other mod.

This was done on purpose so they can never worry about being shut down or sued if they ensure they never wrongly re-use or provide copyrighted materials. Probably a smart move for such a massive undertaking.