Jason had used the funds he managed to raise from his racing outings to survive while once again trying to start professional football.
Eventually, his efforts paid off, but due to his less-than-average skills at the professional level, he always had financial issues so racing had been something he had stuck to doing for most of his life even as he traveled to different places around the world.
He had to make ends meet and anything past street racing was way too illegal and dangerous for Jason.
He could have used his martial arts skills to join underground fighting rings, but doing that was very dangerous and he could end up sustaining serious injuries.
It was also not something he could hide as easily as his involvement in street racing, so he had avoided it.