"Hoopy" is hitchhiking slang, and we don't see many hitchhikers talking among themselves
There is actually one sentence missing that leads to the "Hoopy" quote. The full quote from the books is this:
Hence a phrase that has passed into hitchhiking slang, as in “Hey, you sass that hoopy Ford Prefect? There's a frood who really knows where his towel is." (Sass: know, be aware of, meet, have sex with; hoopy: really together guy; frood: really amazingly together guy.)”
It's clear that this sentence is supposed to be hitchhiker slang, and we simply don't see many real hitch-hikers who would talk among themselves in slang in the novels.
We see Zaphod using the word "Frood" once, but it would not be out of character for him to emulate slang in order to sound cool:
"Frogstar World B, and man it's a dump", said Zaphod running onto the bridge. "Hi, guys, you must be so amazingly glad to see me you can't even find words to tell me what a cool frood I am."
"What a what?" said Arthur bearlily.
(The Ultimate Hitchhiker's Guide To The Galaxy, Chapter 13)
We see Ford using the word Hoopy, but not in the sense the Guide defined it. The most likely explanation for this is that due to his time on earth, Ford is also not up to date with the latest Hitchhiker slang. Or, as people have pointed out in the comments, he could be using it as the hitchhiker's equivalent of "Dude!"
"This one's a dead hairdresser. Hoopy!"
(The Ultimate Hitchhiker's Guide To The Galaxy, Chapter 23)