I had trained Karate in the traditional Shotokan style for 5 years and made it to 5th kyu (purple belt) when I (then 26) found myself in a self-defence situation that unfolded completely different to what I had always imagined:
I had been visiting a friend in an apartment complex and was just about to unlock my bicycle when a car driver thought he had to park right where I was standing. He must have been in a hurry and drove with verve into the parking space, almost hitting me. I let out a startled “Hey!” and “Watch out you idiot!”.
The man, about a head taller, a little older and a lot heavier, got out and challenged me with a threatening undertone to repeat what I had just said. I did, still full of adrenaline.
With an angry stare he walked round his car towards me. I remember thinking “oh, this is going to be interesting” and assumed a relaxed yet ready stance.
At the same time, I was determined to use my strongest technique – a mae geri chudan (deep kick) – without warning as soon as he invaded my personal space. I would not let him grab me and use his size and weight advantage.
The man slowed down as he approached me, as if my ready stance was irritating him. He stopped a few centimetres outside my personal space and clearly didn’t know what to do next.
I said you scared me to death with your driving style. He then apologised! If he can apologise, so can I and I said I was sorry for calling him an idiot. All of a sudden the tension seemed to evaporate into thin air, he turned round and walked away without another word.
“You win 100% of the street fights you never get into.” (Doug Armey)

There are several reasons.
First, not all black belts are created equal.
I trained in Shotokan Karate, a traditional style, taught in the traditional way.
My sensei was a national champion and highly respected teacher.
He believed that if a person attained black belt that person should truly be a black belt in ability.
I trained intensively for 6 years to attain my 1st degree black belt. And a total of 12 years to attain my 3rd degree black belt.
There were other dojos in our city where you could get a black belt in 1 year.
I watched some of those “one-year-wonders” spar with our middle rank students and get their asses kicked.
So there is a vast difference in black belts and their abilities.
Second, some dojos and teachers focus only on point sparing and formal kata forms to be used in competition.
My sensei focused on competition but also on self defense. We trained equally in both.
We had students who were very successful competitors. Yet, they also knew how to apply the techniques in real life on the street situations. And on occasion did successfully.
Third, street fights are designed by the attackers to not be a fair fight.
Often on the street you will face multiple attackers and they will have weapons. And there are no rules to protect you from serious injury.
My karate training was helpful in that as well.
We were taught how to avoid needing to use karate on the street. And our students after sparring with real black belts realized there were some really good fighters around who you did not want to ever go against for real.
Also, we emphasized an important life principle:
“You win 100% of the street fights you never get into.”