Why is death is often personified in literature?

It depends on which culture you're talking about. It's personified as nothing in most western countries. A void of eternal darkness where life ceases to exist in any form, even in an afterlife. Meanwhile, Spain has The Day of the Dead which is meant to celebrate the afterlife and the people in it. Death means different things to different people because there's no real, tangible way to know what death is until it happens to us.

This personification appears in many literature classics as well as poems from 15th century onwards in Europe. As a general rule, this personification is intended to be scary and frightening, so that people can have the proper respect for Death, and value their own lives while it is in process.