They're not, Lee just wants to be like Gai xd
From the Wiki:
During his time as a student in the Academy, Lee proved to have no talent for ninjutsu and genjutsu. As he was mocked by his peers because of this, Lee persevered, focusing on taijutsu.
When Lee eventually graduated he was added to Team Guy along with Neji Hyūga and Tenten. During the team's first meeting, Lee vowed to become a powerful ninja without using ninjutsu or genjutsu. Neji laughed at him, but their sensei, Might Guy, took a special interest in Lee. He encouraged Lee to keep at it and, in time, began teaching him powerful forms of taijutsu.