The spontaneous laughter that comes throughout the day comes for a few seconds here and there. If you add up, it does not make even 5-6 minutes in 24 hours. That is not enough to bring physiological and biochemical changes for health benefits.
Moreover, laughter that comes by using humor is not always positive. It involves sarcasm, ridicule and at times it is vulgar and offensive. Humor is based on individualism and different cultural traits and there is no guarantee how much we can laugh.
In contrast, laughter in Laughter Yoga is unconditional and we laugh with other people for the pure joy of it. In order to get scientifically proven benefits of laughter one needs to laugh for at least 15 to 20 minutes with breaks of relaxation in between. That is only possible by doing laughter as an exercise.