Carrying large quantities of cash across a border is usually investigated by the border guards. If you don't declare it, you're breaking the law. This is compo

Carrying large quantities of cash across a border is usually investigated by the border guards. If you don't declare it, you're breaking the law. This is compounded by how some countries are big enough that traveling across a border is a fairly significant undertaking (Brazil, for example). Not something most folks can or will do on a whim. Plus, you'd have to transfer a fairly significant amount of money to be worth it, making it more likely you'd run afoul of the border patrol.