It's a risk mitigation measure. The three or six month requirement usually comes from how long you could stay, not how long you do stay. You are also assuming

It's a risk mitigation measure. The three or six month requirement usually comes from how long you could stay, not how long you do stay. You are also assuming a point-to-point trip. While a citizen of a country usually cannot be denied entry to their own country, any countries you transit are under no obligation to allow you through on an expired passport. tl;dr filters out people who may be problematic to deport.