Why are there so few democratic governments with multiple rulers?

What I mean is most governments with multiple rulers are tribal communities, dictatorships, and monarchic coalitions.

Governments with multiple rulers are duumvirates/diarchies, triumvirates/triarchies, quadrumvirates/tetrarchies, and quinumvirates/pentarchies.

So, why are there so few democratic governments with multiple rulers (head of states and/or head of governments)?