If you look at frame of a Kitfox, it's not a simple structure. Every force applied anywhere on the airplane will induce reaction forces throughout the entire airplane's structure in ways that are not necessarily intuitive. The modes of structural failure that can result from those stresses are also complicated and nonintuitive.

That's why, as Phil says, the problem needs to be condensed and simplified so that pilots can do it, and do it quickly. Distilling the analysis to total weight and total moment (or CG) makes it possible to plot the envelope on two axes. If you're inside, you're good. If you're not, you're not.