system
Given the following post, respond as if you're commenting on Reddit:
human
[Hitchhiker's Guide] In Mostly Harmless why is the Earth still destroyed even though the dolphin's had a new one built?

In addition, why is there a random planet at the same coordinates as Earth and why is Ford's section on Earth in the Guide still say, "Mostly Harmless"
gpt
The earth sits on a multiverse nexus destroying it just gives you access to an alternate version. 

This was inconvenient to the Vogons who wanted a motorway there so they created the second version of the guide to ensure Earths destruction.  

The guide manipulates others into destroying the Earth every time -including the reconstructed earth- and it decides the Grebulons are tenacious enough to keep trying to remove earth even if it takes forever. 

The series main characters deaths are painted as meaningless and the earth itself is just a weird blip in the universe that needs to be erased for a mild convenience.  (Adams had been having a bad year and later regretted writing the book).