Not all knife fights involve two people with a knife.
In fact, most knife fights have only one person with a knife. For stage, both people having a weapon adds tension and makes things interesting. In reality, someone who knows how to use a knife knows that it is extremely rare to escape without getting seriously cut, even if they win.
Sometimes you will see an unarmed vs. knife fight onstage, often with the hero overcoming the armed villain, which is also quite unrealistic. You usually don’t get a chance to defend yourself. The attacker will want to stab and kill you before you are aware you are in danger. If you know what’s coming then the attacker now has a change of being harmed, which is the last thing they want. Even then, knife beats flesh every time (and forget trying to impaled your free hand on the knife to take it away!)
Two people with knives is just another way we bend reality to tell a good story. Two people with knives are (often, but not always) more interesting than one guy falling down bleeding.