Plot hole in The Hitchhiker's Guide to the Galaxy?

It's been a while since I read those books, but I don't think it's explained in any of the later ones.

The simplest explanation is that Ford Prefect and the editor of the Guide didn't know that Earth was a computer. Bear in mind that it was built 10 million years ago, and the Magratheans (who built it) have been in hibernation for the last 5 million years, so there probably aren't many people around who still remember what Earth is/was for.

(An even simpler explanation is that Douglas Adams made up everything in the radio series (on which the first two books are based) as he was going along, and had no idea when he wrote that the Guide describes Earth as "mostly harmless" that it would later turn out to be so important.)

As the book notes, "The Hitchhiker's Guide To The Galaxy" has many omissions and is wildly inaccurate.

The earth was one of millions of minor insignificant planets in the galaxy and the Guide can only handle so much information. Any importantance the Earth might have held in the grand scheme of things has been fogotten and lost in the history books. Besides, since nobody on Earth ever bought a copy of the Guide, why would the publishers bother with such time wasters?

"Mostly harmless" refers to the inhabitants, not the planet itself.