My guess is that it's because of emergencies. If you get injured and can't or shouldn't fly home then you need more time on your passport. It is also much easier to send you home if you over stay your visa if your passport is still valid. Also the system is setup to give you a visa of a specific length (eg 30 days), they can't just give you a 2 day visa.

Also if your passport lasts for 10 years you've known when it's going to expire for quite a while, they're just expecting you to be responsible.