Because the random roles were still there to account for it. That's why that's what people are trying to use to hide in. But once their are 4 people claiming what has to be a random role, you have a wolf in sights. How do you have 4 wolfs using 3 random spots to hide?
That's why the wolves can't just hide as random villagers. They have to fight for the fixed roles, and especially for the seer roles. And the fact that the random roles CAN BE fixed roles means that if you accidentally kill an aura, there's still no guarantee that you have found a wolf. The other aura might be real also.
Now this obviously becomes harder once the random villagers are all proven (either by dying, or by being provable, or by being checked by the only seer). Now you can still fight for the fixed roles, but the town will know that it's a 50-50.
But the game starts as 9 vs 6. You only really need to mislynch ONE villager to put the game on a knife's edge (next day, 7 v 6). That's it. You could even go absolutely balls to the wall day one and have every wolf claim to be a seer of some kind and all of them say the same villager is evil. It would probably work and evils would win! Because even though the next day the town will know all of the evils, it's already too late especially in a game like this one where there are no bullets.
Let's say day 1 in this game you have a wolf fake spirit seer one fake aura seer and one claim something like red lady who visited the fake spirit seer and the last wolf claims the role of whoever you are deciding to sell. So lets say they decide to sell Benzine:
Wolf 1 as Spirit Seer: Benzine and Holly are RED
Wolf 2 as Aura Seer: Benzine is Evil
Wolf 3 as Red Lady: I visited Wolf 1 and they are not a killer.
Benzine claims Conjuror.
Wolf 4: Benzine is fake I'm the real Conjuror.
So they all sell the shit out of Benzine and I can't imagine Benzine possibly surviving..
Now it doesn't even matter that all wolves are known. The wolf shadow is unstoppable the next day. Another villager gets lynched. The shadow the next day is even more unstoppable. It's 3 villagers vs 4 wolves,1 instigator and 1 instigated townie. The game is over.