why do road frames (and Ritchey mtbs) have/had level top tubes?
Road bikes need to be sized on head tube length. That length determines a window of handlebar height and saddle to handlebar drop adjustment. The horizontal top tube running from top of headtube leads to seat tube and determines length c-c. You chose frame size based on leg length and this determines head tube length. It's this number and not top tube length what was used by italian builders to chose size for the riders.
All this is gone due to new headsets, stems etc.. it worked when quill stems ruled the earth.