The 10 year ceasefires are because Paradox likes to make it impossible to take out an enemy in a single war. Each war can only take a bite of a certain size, t

The 10 year ceasefires are because Paradox likes to make it impossible to take out an enemy in a single war. Each war can only take a bite of a certain size, then you have to wait a while (during which they can rebuild their military) before you take another bite. In theory it means a loss isn't catastrophic, but in practice if they can beat you once they can beat you again, your only real hope is to find allies--and when you're weak it's unlikely you'll find allies.