Well, the combos are a part of the game, and to be a top player you need to know how to execute your characters combos. But for real competition, (at least for SF, and smash bros, never watched competitive MK), it really comes down into anticipating, reacting, and adapting to your opponent. You need to know what options your opponent has available to them, what are they likely to do, and how to properly react. You do need to memorize the timings of attacks, but the game becomes sort of a fast paced unbalanced rock papers scissors. David Sirlin wrote lots of articles and even a book about competitive gaming in general, specially fighting games, explaining concepts like this.
It's just that some games have this strange curve in the learning curve, for instance Starcraft, where beginners think the game is all about strategy. But as you learn to play, you find out that it's more about macro, cause having more units normally beats any strategy. But then if both players have top notch macro, they have to begin doing tradeoffs in what they build, maybe cut some units to get a technology faster, or be greedier but more vulnerable for a while for a later payoff. In other words, the game becomes about strategy.
It's just that some games have this strange curve in the learning curve, for instance Starcraft, where beginners think the game is all about strategy. But as you learn to play, you find out that it's more about macro, cause having more units normally beats any strategy. But then if both players have top notch macro, they have to begin doing tradeoffs in what they build, maybe cut some units to get a technology faster, or be greedier but more vulnerable for a while for a later payoff. In other words, the game becomes about strategy.