-The weight and intensity of unarmed training are not enough to stimulate muscle growth.

-the movements of unarmed training are not diversified enough to fully exercise the muscles of all parts of the body.

-the time of unarmed training is not long enough to achieve enough training.

-The diet and rest of unarmed training are not reasonable enough to provide adequate nutrition and recovery time.

Of course, these reasons are not absolute. If you can scientifically and reasonably carry out unarmed fitness training and maintain good eating and rest habits, then you can also build big muscles!