Stupid Premise Which Likely Never Happened In The Old West
Two people equally skilled pitted against each other have no surety that one will always win.
Happens in the sports world all the time.
They would not risk their lives just to be known as the best. On any given day, the 'best' can lose to someone less skilled, let alone equally skilled.
The results of such a gunfight only proves that one was the best on that particular day. No one is going to risk everything they have or ever will have to prove something so small.