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.”