Because they’re dishonorable. Phrased that way, it might sound kind of stupid, but that is the logic behind why these get treated the way they do. Another example would be shooting, or stabbing, someone in the back, or gouging out an eye.
Also, these techniques aren’t created equal. Some of them will legitimately end a fight, others will just annoy an attacker.
Tearing a chunk out of someone with your teeth is a very effective way to convince them not to mess with you. Biting has some serious social stigmas associated with it. Human bites are very dangerous. The bacteria in your mouth are a major health hazard to anyone you play Pacman on. This is before considering that you can deliver a lot of force with your teeth. Your mouth is designed for tearing apart meat, much like the meat found in your foe’s arm or neck.
Biting someone can kill them. It’s also not a great idea, because you’re exposing yourself directly to their blood. If they have a blood communicable disease, then there could be serious health implications for you in the future.
Scratching someone doesn’t really work. In most cases, it will annoy them, at best. Even with long nails, the only damage you’ll do is superficial. One rare exception is if you rake the eyes. But, honestly, targeting the eyes is another underhanded tactic.
Pulling hair is iffy. It gives you some control over the head (if you hold on.) Unfortunately it also puts you very close to your opponent, and if depending on their tolerance for pain, it ties up one of your hands for limited advantage. If you’re using their hair as a tool for bouncing their skull off a hard surface, then it is an option, but on its own, this does nothing but irritate your opponent.
Tagging someone in the groin will piss them off. Depending on the individual, it might not do much else. Pop culture will teach you that this is an instant off switch for any guy. Reality will sometimes shrug off the blow, and retaliate. It will hurt, but connecting with enough force to incapacitate is not a certainty.
A lot of our concepts about what is acceptable behavior in a fight is inherited from historical rules of engagement. Rules which were originally designed to prevent a medieval peasant from accidentally killing a noble instead of capturing them for ransom, still filter up to the surface today, in prohibitions about killing someone without seeing their face.
In very general terms, dirty fighting is anything that undermines the rules of “fair fight,” and the general idea that combat is about a direct physical contest between two opponents.
It’s important to understand that concepts like honor and dirty fighting are defined entirely social norms and values. Social norms dictate what is acceptable behavior in combat. There’s nothing intrinsic about any of these techniques that make them evil, or particularly underhanded, but our society says, “don’t do this, it’s beneath you.” So, it’s “dirty,” because social norms said so.