The redoubtable Melvyn Quince waxes philosophical.
It seems to me that a dictionary should be there to provide entries for the things one could not know: who would have thought that rhubarb would have the name rhubarb? It’s so surprising it’s almost incredible. If you had asked me to guess, I would have been picking it out of millions of possible words of that length. So it goes in the dictionary. But I think I would have been able to guess that the main Libyan desert might be called the Libyan Desert.
But leave out pieplant?