Why are there so many Asian martial arts but not that many from other cultures?

Krav Maga out of Isreal, and just about every tribal culture has there own style of fighting. there is more of a presence of martial arts in asian cultures due to how long the civilizations have existed and sheer numbers alone. you have to figure at the point when romans were holding their gladiatorial arenas, the mongols were storming across the plains and steps of asia fighting constantly.