They're Young, They're in Love... They eat LARD
if i have no more dice i can load no cargo on waiting ships even if i have a contract?
feels very unthematic
there should always be cargo
If you require a thematic reason consider the 9 available dice as an abstraction of your company's available containers.
Beyond the reality that no company has infinite access to materials, Resources are finite for balance reasons...with unlimited containers players would potentially find themselves in a death spiral of containers migrating to the warehouse. The balance of 9 containers per player is a logistics puzzle that is challenging, but manageable.