Demand of Supply
When Galifar I united the entire continent of Khorvaire under one ruler, the Dragonmarked Houses met with the would be king in Korth and make arrangements that would limit their political power in exchange for assurances of their continued economic strength. These would last for a thousand years and were designed to prevent members of any of the Dragonmarked Houses from creating standing armies (excepting House Deneith to provide mercenary services), disallowing members of the houses from marrying into the aristocracy of Galifar, and insuring that no house could hold a grant of land.
A few exceptions to the edicts exist today including the Valenar elves indifference to them, and everything that occurs outside of Khorvaire. Some within each of Dragonmarked Houses believe that they should be done away with since the empire of Galifar no longer exists.