Maybe you're right and Allegri should be fired, but management still trusts him. Maybe they are waiting for Juventus to play in the optimal composition and hope that everything will change for the better in this case? A rather risky assumption, since if it turns out to be wrong, the next coach can get Juventus in a situation where there will be no chance of reaching the playoffs of the Champions League, and in Serie A the gap from the leaders will be critical.
It's very ironic, when he joined Juventus first time, he was able to show his coaching skills very well. Serie A title, he can achieve it in few season. But, after he quit and rejoined, Juventus did not improve. Not entirely Allegri's fault, because the composition of Juventus players is still not entirely good for him, unlike when Allegri coached Juve first time. Next season probably become Allegri's last season at Juventus, if he get another season, but it can't improve Juventus.