Well we could say that it saves weight, which is true, but the honest answer is that it makes them easier to to produce and so reduces the price. There is simply no need for full depth splines. If a kickstart gear is going to spin, it must shear the splines at the root. This will happen regardless of how deep the spline is. Remember that this gear is only used by the kickstarter. Obviously it is vital that these splines are strong enough to withstand the kickstart pressure. Would they be stronger if they were deeper ? No they would not. The strength of the splines in shear is dependent only on their width, not on their depth. Our splines are 0.5mm deep. O.E. splines are about 3mm deep. If they were 10mm deep, it wouldn't make them any stronger! The splines on these baskets, being made from 7075 alloy, will withstand more than twice the kickstart force of O.E. ones! This diagram illustrates how the forces act on the kickstart gear splines.
As you can see, the depth of the clutch basket spline has no effect on its ability to resist the force of the kickstart gear.