Fights are unorganized
Fights are not choreographed as fights in films, nor are they as organized as Taekwondo sparring matches. Fights are crude and rude. As martial artists, we work to perfect our techniques, but in a fight, the bad guy is not impressed by technique, he or she is only impressed by pain. A technique that is very easy to execute in class against a willing partner may be next to impossible to execute in a fighting situation. It is difficult to hit specific targets and complex combinations fall apart. As martial artists, we practice techniques against other martial artists, so boxers get good at fighting boxers and Taekwondo students get good at fighting other Taekwondo students. To win a fight, you must train to fight against an un-classically trained person who is probably well experienced in street fighting.
High kicks are unlikely to be effective
High kicks in a fight are dangerous to the kicker. First, your clothes will probably not permit the movement required. The soles of your shoes may grip the floor/ground too much or they may slip and slide. Your leg may be grabbed leaving your vulnerable. Groin kicks are expected. Street fighters know this so they only kick at the knees.
Real fights are not like sparring matches
There are no rules or referees in a fight. Your opponent can bite, gouge your eyes, spit, use weapons, etc. Losing a fight can result in permanent physical or mental damage, or even the loss of your life. You must be prepared to use violent, repugnant methods when it becomes necessary.