Problem is, one "model" of pants comes in a "grid" of waist sizes and leg lengths. Typically, one model of bike comes in a "linear" selection of frame sizes, with the seat tube and top tube lengths increasing proportionally. To get the right seat tube / top tube combination, you either have to shop among different models (potentially compromising on quality, componentry or features) or go custom-made. So yeah, it's a lot more complicated than simply going by overall height. It's even more than just upper-body vs. leg length. Ratio of upper-leg and lower-leg length determines crank length, seat height, fore/aft seat position, stem length, stem height, etc.