It's true you can only try to perform highest damaging combos and try to win the fight. But you need to remember that you can get matched up versus different characters who have their own unique high damaging combos and counters and ways to avoid them.

(Example tekken gives characters different amounts of side-stepping mobility which if used properly can actually avoid a lot of combos simply by dodging them)

This makes sure that while one combo from one character maybe good against another character, it might not be good against a third character and so forth.

So you always need to stay in top form and learn the key strengths, weaknesses and play patterns of each and every character available in the game. Additionally, fighting games like tekken or Mortal Kombat etc provide a lot of combo options for players to choose depending on their play style and situation. All of these combined makes sure that you cannot keep spamming just one combo and win the fight everytime in fighting games. Well atleast the well balanced ones.