Stems normally hold the bars forward of the fork for the same reason airplane wings slant upward.

That orientation imparts a natural stability and tendency to settle to level (straight ahead) when not pushed otherwise.

However, just as there have been successful airplanes with wings tilting down, nothing, except seat to bar distance considerations stops you from reversing the stem.

Odds are you'll need a stem with minimal extension, and be sure to allow for the change in rise when reversed.

Once done, expect the handling to be a bit more skittish until you adapt.