Speaking castles...

I donn't understand one rule on them. Why do they stop troops from moving
through provinces?

I can understand the rule about a province with a castle not being
conquered/occupied until the castle is destroyed/neutralized...but why can't
you simply move troops through the province? Castles are so massive that they
occupy *entire* provinces (which are, like, what? 1,500 square miles?) Could
someone clarify this for me?

- - The Motive