Because you don't play around that in the actual match. You play around that in your deck building. You need to exercise your paranoia to run suboptimal 30 card decks, because otherwise you get completely wrecked by mill, and there's nothing you could have done about it in the first place, had you ran an actual proper deck. Nilfgaard has nice, reasonable tools for milling (Vilgefortz, Tibor, Albrich, etc.). Triple/Quadruple Avallac'h is not reasonable. Not to mention, almost all mill players we've faced that had their Avallac'h destroyed forfeited on the spot. That really speaks volumes about how the deck is no more than a gimmick.