Playing a midfield as attacking as Real's one is just so dangerous.
Not only can it lead to the concession of counter-attacks, there is also the problem of not having enough control over the midfield to consistently get the ball to the forwards, which can render it rather pointless loading the team with attackers.
There has to be a chance of Real's lack of midfield control leading to an open, chaotic game that would suit Real more than Juve. Juve have to be very careful not to get drawn into that sort of game.
They can allow it to open up up to a point, but always being sure to keep enough midfielders behind the ball to provide a level of security and to guard against the game opening into simply being attack versus defence, rather than being played in midfield, which is obviously where Juve's strength lies.