Because you will break the center-T on all trained kites.
All sport kites when flown by themselves are supported at 5 points.
So when you train them, the center-T of each kite must be supported,
Not positively so in every case.........We have a ten stack of Microns (second generation) and only # one is supported at the center tee because of the bridle........All nine others are linked with four lines apiece at the upper and lower spreader attach points........Have flown it in 20 MPH winds and nary a problem.......I agree though that heaver kites do need more trussing for to keep them from wobbling and the inevitable death spiral that would result.
The Micron stack comes from Prism with a center tee stack line. The Prism website pictures show the center tee connected.