Of course, theater can indeed hone a person. However, it consumes a lot of your energy, invades your personal space, and for six months to a year, you won't be able to detach yourself to shoot films or TV shows.
Moreover, the reach of theater is very limited, and it's not easy to turn it into a film or TV show, that requires separate production. The only audience you have is the few thousand people in the seats, so what if you perform well?
That's why no popular stars go into theater. Unless they're stuck in their acting career, or they want to make a comeback, and they dive into theater to train, hoping to seize a role and perform excellently.
