There's many dramatised portrayals of military leaders challenging their opponents to single combat in place of a full-on battle - from Game of Thrones to recent films like Outlaw King and The King - but are there any examples of this actually happening? Why wouldn't the opposing forces just go to battle anyway, regardless of the fight's outcome?
No, there are no historical examples of this actually happening, because as you say, there would be nothing stopping the opposing sides from just going to battle anyway. In myths and stories where these duels were fought, the outcome was always disputed, leading to re-escalation of the conflict. The only maybe historical example is the so-called Battle of the Champions between 300 Argives and 300 Spartans, which also ended in a disputed outcome and a full-scale battle. I wrote about this in more detail in an older thread.