Why does quitting your job early to start a business increase your chances of success?

Because a person's ambition is more valuable than ability!
When you're 5 years old, you want to land on Mars and dominate the universe;
At 10, you want to become the President of the United States, because every child has the potential to be the future U.S. president;
At 20, you want to be a CEO, marry a wealthy and beautiful partner;
What about at 30? Maybe you just want a fully paid house, a fully paid car, several million in savings to invest, and to relax earning interest.
A person's ambition diminishes as they grow older.
By the time you reach 35 and start a business, do you still have that drive to accomplish great things? Do you still have that execution ability?
Probably not, you just want your wife and kids to be happy at home.
If you're currently between 24 and 30, remember, this is the last window where your ambition is strongest, where you need to take risks, and where you're least afraid of failure.