Several reasons but one is that, a lot of games where cheats are super effective are places where you can have "silent" advantages like ESP, equipment data, silent aim adjustment for better headshots/tracking, etc. These rely on the ability to not be detected and are extremely subtle when compared to normal gameplay. The more subtle, the harder to detect. Little of this applies in a fighting game. It's largely a mind game, not a game of inputs or technicalities. You're on the same level, can see everything. The only hidden information is what moves/combos you have, your opponent has, and how much information your opponent has on you. You have to be able to read your opponents moves, and respond appropriately. You have to trick your opponent and bait them into a scenario that is in your favor. Attacks have definitive strengths and weaknesses, and certain baits are better against certain people. If you attack me, and I intercept your attack with a "crushing blow" that simply overpowers your attack -- there's nothing the cheat can do to stop that, short of just like, breaking the game and undoing my damage. If I bait an attack, and you do it, and I dodge it by laying on the ground -- it can't retroactively change your input. If I set you up for a combo and begin wailing on you, the cheat cannot generally stop that. These games have mechanics where certain inputs just can't follow other inputs, where inputs are buffered certain ways, where there are vulnerability windows, etc. Exploiting these are key to success and cheats can't stop these mechanics.
An example is Tekken where I play as Lei Wulong. He's extremely uncharacteristic and idiosyncratic, to the point that if you fight actual Lei players -- it's immediately obvious that bots don't play the same. One of my friends (a newer fighting game player) particularly hates my playstyle and tried to scrim against Lei bots to practice, but instantly realized they play nothing like me or any other Lei. Lei is a rare character to play. So he had to just keep scrimming, really.
Many fighting games also have literal handicap mechanics that might be seen as equivalent to cheating in some capacity, but for the same reasons, it doesn't matter. They are often there to level the playing field, which is considered fair. Basic Combos in Tekken allow even the simplest of players to pull off powerful moves like Wind God Fist, but it doesn't matter how easy Wind God Fist is, I can and will still beat them even with that handicap, with no special moves of my own, because I can just read their attacks and respond and punish all of them and set them up and bait them endlessly.
That said cheating does happen but often it's the last thing on my mind. Bullshit characters that are unbalanced is where all the complaints go. ;)
TL;DR Humans have distinct fighting styles, and fighting games are largely mind games once you get into them. Cheats that are subtle enough to avoid detection can often easily be outplayed because of it.
