For most people savy enough to use an arbitrary answer, it is because you can use a different one each time (with the very few exceptions where you may not be a

For most people savy enough to use an arbitrary answer, it is because you can use a different one each time (with the very few exceptions where you may not be able, e.g. banks). Offline portable password managers like Keepass/Password Safe/etc. are immense help for these non-bank services.