He's captain, one of the oldest players in the team and an absolute pillar of the starting XI. Such players easily deserve 9-10m net in an elite club. Laporta reduced the wage bill to a bit over 50% of the total ordinary income, which is a top level among the elite clubs, meaning Barca can easily afford this salary.
Also this is his last contract in his prime, for the next one he'll already be 31-32 declining. So obviously he wouldn't accept a much lower salary.
Or the other way around, try to find a player like Frenkie, that costs the club 20m/y in total (fee + salary).