In a simple word

... loft. The shaft flexes forward at impact which increases the loft of any club. With a shaft sufficiently stiff as to provide minimal loft, it would be very difficult to get the ball air-born. It is the selection of club head loft angle AND shaft stiffness which are used in conjunction to get players to their ideal launch parameters. Shaft stiffness ratings are relative. A "stiff" shaft is still flexible enough to add significant loft to the club/shaft equation. A faster swing speed will induce more flex in the shaft and so stiffer flexes are used in these cases to rein in the added loft that results.

Take either shaft flex or club head loft angle to near zero and you have a very difficult club to use for anybody. It's interesting that someone has proposed using an infinitely stiff shaft to "help" us, but nobody would consider a driver with zero degrees of loft as being helpful.