Listening to the @NotACastASOIAF I realized the story of the brave companions explains why essosi sellswords don't usually cross to Westeros. It is because only the war is over they have no way back, and they are in fact surrounded by enemies.
La trilogía de secuelas de Star Wars debería haberse basado en el universo expandido. Los villanos deberían haber sido Thrawn, Darth Krayt y/o los Yuuzhan Vong.
A proof of that is that it rarely acted as a cohesive region in the different civil wars until Robert's rebellion. Different from, for example, the North where the oldest house was maintained in power.
Aegon did a great move by naming the Tyrells Lord Paramount of the Reach. Being a house of Stuarts seen as lesser by the older houses they would never unite the whole region cohesively against the throne. The Reach being clearly the most powerful kingdom.