“ Why do Republics/democracies not have Revolutions?”
This is a pointless question. Did the Civil War not take place?
Most dictatorships, like Mexico in the 1830s and Germany in the 1930s, start off as republics/democracies and subsist until someone decides they do not want to give up power. Even now in the US, we find that elections may not mean much because the Deep State can rule without regard for the person chosen by the electoral process. Can revolution be far away in such an event?