Glass isn't an extremely viscous liquid, it doesn't flow over long periods of time. Old medieval windows are thicker at the bottom because glass making was not precise enough to make sufficiently flat pieces back then, and pieces to be installed were always installed thick end down.
Or more exactly the process was entirely different from how large panes are made starting from 1950s... They were essentially blown and flattened like other glass items. Where as now we float massive continuous sheets over molten tin, thus having uniform thickness...
